As posed in the headline. Many books did not make it into the bible as decided by the church. But do historians who use the bible as an historical document give equal weight to these other books or are they considered less reliable because the early church felt that way? Or as a result of the church's decision, are those copies we have today considered less reliable because they are less documented? How do historians feel about these books, like the gospels of Mary Magdalene and others?
Historians generally approach them on the same footing, that is neither books in the bible (canonical texts) or out of the bible (non-canonical texts) are assumed to be any more ‘historical’ or ‘reliable’ because of that criterion. Canonical status is a theological value of religious communities, it doesn’t bear on the historical reliability of a text itself. It may have some bearing on how that material was preserved, and so how many copies of it we have, what sort of textual history it has, etc., but in terms of the primary document itself, no effect.
Each of these primary documents has to be evaluated on its own merits. That said, one of the reason that quite a few of these texts never entered the canon of the churches, is that they were written latter than the other documents they claim to emulate. For example, most of the non-canonical gospel texts are dated later than the 1st century. For example, the Gospel of Peter in the second half of the 2nd century, the G of Nicodemus appears to be a 4th century text, and so on. The dating of these texts to later periods means they are unlikely to reflect historical data about 1st century Judea, but they are historically relevant for study of the religious communities in which they arose and in which they functioned as religious texts.