It is hardly disputed that in the whole vast literature of witchcraft, the most prominent, the most important, the most authoritative volume is the Malleus Maleficarum (The Witch Hammer) of Heinrich Kramer (Henricus Institoris) and James Sprenger. The date of the first edition of the Malleus cannot be fixed with absolute certainty, but the likeliest year is 1486. There were, at any rate, fourteen editions between 1487 and 1520, and at least sixteen editions between 1574 and 1669. These were issued from the leading German, French and Italian presses. The latest reprint of the original text of the Malleus is to be found in the noble four volume collection of Treatises on Witchcraft, "sumptibus Claudii Bourgeat", 4to., Lyons, 1669. There is a German translation by J. W. R. Schmidt, Der Hexenhammer, 3 vols, Berlin, 1906; second edition, 1922-3. There is also an English translation with Introduction, Bibliography, and Notes by Montague Summers, published John Rodker, 1928. [...] In fact, "it fastened on European jurisprudence for nearly three centuries the duty of combating" the Society of Witches. The Malleus lay on the bench of every judge, on the desk of every magistrate. It was the ultimate, irrefutable, unarguable authority. It was implicitly accepted not only by Catholic but by Protestant literature. In fine, it is not too much to say that the Malleus Maleficarum is among the most important, wisest, and weightiest books of the world.
You have to take Summers with a grain of salt: while extremely erudite in the subject of the occult, the Reverend's actual belief and tendency toward exaltation of occult literature can skew the perception of facts a little. Nevertheless, it does emphasize that the Malleus was extraordinarily popular, and for an extensive period of time. A lot of this had to do with utility and demand.
If you look at the volumes available to authorities on witchcraft in the period, you have works like Johannes Nider's Formicarius (1475); while this was popular and helped define the image and concept of the witch as we know it, this work was not geared toward either a lay audience or legal use since it was quite literally written as a teacher-student dialogue on the lives of ants. The section on witchcraft was actually absorbed into the Malleus.
And authorities at the period needed a work on heresy and witchcraft. Civil courts and authorities were under greater pressure to work with ecclesiastic courts and authorities to address both heresy and maleficium, and the framework of Roman Law which was highly influential on the courts of Europe required a certain amount of legalistic wrangling to deal with the contemporary issues of witchcraft. While the Protestant Reformation had not yet occurred, there were still schisms within the church, which inquisitors rooted out as heresy. H. R. Trevor-Roper in The European Witch-Craze of the Sixteenth and Seventeenth Centuries writes:
When the Dominicans pressed for inquisitorial power over witchcraft, the papacy had at first resisted. The old canons of the Church, and particularly the canon Episcopi, denied the reality of witches and forbade their persecution. Therefore, in 1257, Pope Alexander IV had refused these demands unless manifest heresy, not merely witchcraft, could be proved. But little by little, under constant pressure, the papacy had yielded. The great surrender had been made by the French popes of Avignon, and particularly by the two popes from southern France, John XXII and his successor Benedict XII, who had already, as bishops in Languedoc, waged war on nonconformity in the old Albigensian and Vaudois areas. John XXII, who declared heretical the Franciscan doctrine of the poverity of Christ (so dangerously akin to the old Vaudois ideas), also, by his constitution Super illius specula of 1325, authorized he full use of inquisitorial procedure against witches, of who he lived in personal terror. For the next century and a half--until the Witch Bull of Innocent VIII, and indeed afterwards--the main effort of the inquisitors (although there were some spectacular 'political' witchcraft trials in France, Burgundy and England) had been directed against the witches of the Alps and the Pyrenees.
Summers gives a good run-down of some of the "political" witchcraft trials in his introduction to the Malleus; sorcery was effectively the civil counterpart to the ecclesiastical charge of heresy, and civil authorities could actually pass mortal sentences, which ecclesiastical courts could not--this was the standard inquisitorial practice, to hand over heretics to secular authorities for further persecution and punishment. This practice applied to all cases before the inquisition, not just the political ones, so as the scope of the inquisition expanded to include witchcraft, the need by civil authorities to address the question of witchcraft expanded as well. This brings us to Pope Innocent VIII, who in 1484:
[...] issued Summis desiderantes at the request of two Dominican inquisitors, Heinrich Kramer (or Institoris) and Jacob Sprenger, who had worked in southern Germany and encountered some resistance in their search for heretics. The Bull formally removes all impediments to the completion of Kramer and Sprenger's mission.
Peters goes on to note that the Papal bull was not, in itself, remarkable - previous popes had issued bulls with much the same language and for much the same purpose, to authorize members of the clergy and clear away local opposition. However, two years after it was issue, it was included as the preface to the Malleus Maleficarum - a text Innocent himself had probably never seen - and in that context, being bundled with the book, gave the book greater authority, beyond even the area of Germany which most concerned the inquisitors. This in part helped its transmission, but the other part is the nature of the book itself. Peters 173-174:
All scholars agree that the form of the work, that of a series of scholastic quaestiones, derives from handbooks for inquisitors, of which Eymeric's is the best example. Within the framework of quaestiones arranged into three major parts, owever, the work is intellectually inferior to Eymeric's and hardly to be considered "scholastic" in the original sense of that term at all. What is striking about the Malleus is its economic use of the quaestio form to bring together elements from all of the diverse sources that had provided elements of the new fifteenth-century beliefs in magic in the first place. The juxtaposition of these elements in a single, concentrated, tightly argued treatise of considerably greater length than any earlier work really constituted the basis of the popularity of the Malleus during the next two centuries. [...] In no other work are theology and law so tightly linked. Even if, as I have suggested above, the Malleus made no discernible impact on the prosecution of magicians and witches for nearly half a century, no comparable work approached its comprehensiveness until those of Bodin and Remy, Del Rio and Boguet, at the end of the sixteenth century.
The Malleus was not alone in its generation. Writings about witchcraft, even printed works, were extensive, with titles like Tractatus contra Daemonum Invocatores (1450) by Jean Vinet, Fortalicium fidei contra fidei christianae hostes (1467) by Alfonso de Spina, Flagellum Maleficorum (c.1490) by Petrus Mamoris. The Laienspiegel (1509) by Ulrich Tengler in particular was incredibly popular, because like the Malleus it was a legal work looking at the different aspects of witchcraft in regard to private, public, and criminal law; it went through fourteen editions. But the Malleus was uniquely suited to spread, because of its relatively early date, because it was organized in a manner useful to legal authorities, and because the spread of the printing press made it much more accessible to this ready market that was in demand for such a way. Judges and magistrates required a legal text for dealing with the problems posed by accusations of sorcery and heresy, and the Malleus was almost encyclopedic compared to other contemporary texts.
This, combined with the popularity of the Malleus and its relative accessibility also made it something of an authority on witchcraft which many subsequent demonologists would refer to, quote from, or cite as a reference; a kind of self-fulfilling prophecy where a book is popular because it is known to be popular. While there is a popular rumor that it was banned by the Church and placed on the Index Librorum Prohibitorum, I haven't been able to confirm that with any of my sources at hand.
In addition, who was printing it? I feel like the church would have had a bigger issue with people printing it and making it available for sale than with people buying it.
As a follow-up, due to the expense of books, was there much fiction published during these periods? If not, I can imagine it being bought for entertainment purposes - the world's first fantasy novel, in other words. Weren't fairy tales primarily oral?
While it's true that the Church maintained a broadly cautious attitude toward witch hunting, it was not the clergy that the Malleus was aimed at. Instead, Kramer attempted to promote his book among secular lords, princes, courts and town councils. Indeed, one of the central points of the book was that, due the exceptional nature of their crimes, witches should be prosecuted by secular authorities. As no full vernacular edition of the Malleus was ever published, its readership was restricted to only the most educated classes - chiefly lawyers and theologians.
Kramer himself seems to have been something of an anathema in his own time. The 'senile' comment came from Bishop Golser of Tyrol, who had resisted Kramer's attempts to foment a brutal witch persecution in Innsbrook and had him expelled from his diocese under threat of physical violence a year before the Malleus was published. We do not know of a single witch being condemned in Tyrol following this affair. After the publication of the Malleus, Ulrich Molitor, a lawyer who had worked as an official at Innsbrook, penned a critique of Kramer's views that was republished more times throughout the 1490s than the Malleus itself. Kramer himself was ridiculed by Erasmus of Rotterdam in the satirical Praise of Folly, and labelled a drunkard by the humanist Konrad Peutinger. As such, 'widespread acceptance' may be something of an exaggeration.
Nevertheless, by the time the Malleus was published, anxieties about witchcraft were already on the rise, with early witch trials beginning to take place in several European jurisdictions. As a demonological work, the book actually offered little in the way of new ideas, but as a self-purported definitive guide for the handling of witchcraft cases, the book suited the needs of judges and lawyers perfectly. The extent to which the Malleus directly influenced the spread of witch hysteria is still debated, but it seems likely that the book owed much of its popularity to pre-existing fears. The Malleus also received a wider audience amongst secular rulers who harboured a personal interest in the subject of witchcraft or were simply drawn to the novelty of such a text.