If a novel has a glossary, please give some early indication that there exists a glossary. Put it in the front of the book. Or list it in a table of contents (even if it's the only contents to list). Whether the book is print or digital.
As a rule I don't flip through books, and especially not anywhere near the end, for fear of spoilers. Putting a glossary at the end is only useful if you make it that far, but at that point, there's little point.
I remember reading the whole of A Clockwork Orange when I was 18 without the benefit of knowing there was a glossary at the back — sure, I felt pretty smart for being able to figure out most of the terms, but I wondered how readers who didn't know a Slavic language managed, and then I felt pretty stupid for not having realized there was a glossary at the back. Knowing there's a glossary might make challenging books a tad more accessible.
Two recent examples of books I've read that have glossaries I wish I'd known about:
Seraphina, by Rachel Hartman. (I read this book in hardcover.) Particularly as it's aimed at young adults, readers would benefit from knowing they can refer to a glossary to keep certain dragon and other concepts of this fantasy world straight.
Kino, by Jürgen Fauth. (I read this as an ebook.) German words and phrases are scattered throughout the book and add colour. The meaning of single words is generally clear(ish) from context, but I skipped over several of the longer phrases in faith that my not knowing what they meant wouldn't detract from my understanding of the novel as a whole. I'm pretty sure that's true, but now that I've reached the end, I'm not going to flip back to check my German comprehension.
Hey, if the book is digital, why not link the term directly to its definition?