Since a few people are asking this same question, I'll take a crack at answering even though the question is a year old.
My sense is that it means the final book in some series. It could be, for example, the third book in a series. It could also be the final book on a list of books. (Here are six random book titles, I ordered the last one.)
The meaning "final book available" is possible, but less likely. In that case you'd be more likely to say "La lastan ekzempleron" - the last copy.
As for "latest book", my sense is that lasta has this meaning only in relation to time expressions (e.g. la lastan lundon - last Monday). If you wanted to say you bought the latest book, you should say "... la plej lastatempan libron" or similar.