Spain was just a random country due to my admittedly limited knowledge of classic literature, but, seeing as how the whole concept of IP law is (relatively) recent in comparison to civil and common law, how did authors such as Cervantes, Lope de Vega, Shakespeare and the so managed to ensure their work was respected and duly compensated (economically and credit-wise)?
Copyright was kind of sort of a thing, but let me start with theatre.
The standard procedure for a comediographer when he wrote a comedy would be to sell his manuscript to a theatrical company, and that would be the end of it. Different authors had different cachés based on their popularity, just like actors do today. Lope de Vega was very well paid, normally getting some 5,000 reales per comedy, and he could write them like crazy based on formulaic elements, and recycling material. Lope wrote a formidable amount of theatrical pieces, somewhere around 650 plays. The figure is not exact because there are many things circulating under his name that are not his.
In his Arte nuevo de hazer comedias en este tiempo (Madrid, 1609) he claims to have written 473 up to that date, and we the siglodeoristas tend to consider it an accurate figure. If after that date he slowed his rate writing only one comedy per month until his death, you get to over 600, which is not unthinkable for Lope de Vega, the most prolific playwright the world has ever seen. Other writers don't even scratch that number: Luis Vélez de Guevara wrote some 250 plays, Claramonte some 200, Calderón only wrote 110 theatrical pieces (including entremeses, autos sacramentales, jácaras, and mojigangas)...
When the playwright sells his comedy to a stage company, he loses control over it, it becomes a property of the company, and they could tweak it in any way the see fit. Of course, some actors had a better control of their repertoires: Andrés de Claramonte was a company director and an actor, so quite frequently he used his own company for performing his plays. However, as he died in 1626, things start to get confusing with his plays, more often than not getting credited to the authors that were fashionable in any given year: that's why you see the first version of "El alcalde de Zalamea" under Lope's name, same with "La Estrella de Sevilla", "Tan largo me lo fiais" was credited to Calderón, "El Burlador de Sevilla" got credited under Tirso de Molina, etc.
Back to the books. Authors would normally go to printers-booksellers and arrive to an agreement for the publication of their works. The printer would pay them upfront a portion of the expected sales, and then they would keep the rest of the revenue. Cervantes, when he wrote Don Quixote, had asked for permission from the Council of State to print the book, and asked for a privilege of exclusivity for 20 years. This would have allowed him to have his book printed with whichever printer he chose for that amount of time, and in case any printer would countervene that privilege, they would face conficastion and hefty fines. Cervantes eventually got a privilege for 10 years for Castile, Aragón, and Portugal. In Castile, his agreement was with Juan de la Cuesta and Francisco de Robles, in Madrid. In Aragón, he settled with Pedro Patricio Mey, in Valencia. In Portugal he did not settle with anyone, but his book being very popular, printer Craesbeeck published it without authorisation. Cervantes, not wanting to go to Portugal to denounce the man, just let the thing be, as it was not worth the hassle.
What could an author do if he found unauthorised versions of his books, or books printed under his name that were not his? Go to court or to the Inquisition. Francisco de Quevedo once saw an opuscule under his name that he had not written, and he denounced that before the Inquisition, which acted swiftly confiscating any specimens of that book that were still for sale.
Theatre could also be published, either in the form of "sueltas" (a comedy alone, without any fancy things like binding), or in the form of compilations. This is all a kerfuffle as there were many unauthorised versions, with printers-booksellers buying manuscript copies from actors and then compiling a bunch of plays. It gets even worse as from 1625 to 1634 there was a ban on printing theatre in the realms of Castile. What did printers do? Forge the covers. The 1630 edition of "Parte Doze de comedias" was allegedly printed in Barcelona by Gerónimo Margarit, but typographic analysis has established for good that it was actually printed in Seville by Francisco de Lira and Miguel de Sande. If you see any theatrical volume printed in the reals of Aragón between 1625 and 1634, there is a 90% chance that the "house mark" is fake.
Theatre was such a huge business that it had a jurisdiction of its own, established in 1608, with the Ordinances being updated in 1615. Think of it as sports today, with the Administrative Tribunal of Sporting Matters in Spain, or the Court d'Arbitrage du Sport in Switzerland. The "judges of the theatres" would settle copyright disputes, foul play between theatrical companies, illegitimate modifications of texts, etc. There is a fun example I wrote about on this very forum, and which I proceed to link below. It was a dispute between Hortensio Félix Paravicino and Pedro Calderón in 1629, as Calderón had unlawfully inserted half a dozen verses into a comedy he performed at the Royal Palace. Hortensio, being a crybaby, took the matter to the judge of the theatres, and even complained to the President of the Council of Castile, who just about told him to grow a pair.