An epilogue is a bit that comes at the end of a book - but a particular type of bit. It's not the index, which also comes at the end, or an appendix, or the picture credits (which might come at the end), or a glossary (found at the end of some books).
An epilogue is a bit more; it tells you what happens after the end of the story, or sometimes sets the scene for a sequel by giving a tiny bit of narrative that suggests there is a lot more to come. Imagine a story in which the villain is dead and buried, everything is sorted out, you think it's all over ... and then there is an epilogue in which someone wandering through the graveyard sees the earth has been disturbed around the villain's grave. That tells you the story is not over. The publisher wants another £8.99 from you.
In a factual book, an epilogue might suggest some piece of future research or contain an update to the main part of the book (especially if some development happens after the book is ready or nearly ready for press). But it's much better to integrate that into the book, even deferring publication, if possible. With an academic book, the author/publisher has probably been working on the book for years or decades and it represents lots of work that the author is unwilling to throw out just because it's been proved wrong. So the epilogue becomes a type of apologia - a means of explaining why they are still publishing this book after its research has been superseded. It will probably take account of recent research and give some reason (possibly spurious) as to why the rest of the book is still valid.
So an epilogue in a fiction book means 'give us another £8.99 soon' and an epilogue in a non-fiction book means 'that £18.99 [or £58.99] you spent on this book was possibly wasted'.
This post is even more cynical than usual.