I once wanted to find some ebooks of specific authors. A good method to find them is using a search query like:
applied cryptography "index of" "ebook"
If you don't get the righ result you can try to add in the query ".pdf" or change "ebook" to "e-books" etc.
By using "index of" it will increase the likelyhood that you'll be finding real files because these are apache directory listings.
Another good thing about this is that you can use this to find other information about cryptography. For example if you want to know about a certain subject you may have chance that searching for one book in that category will get you to alot more of these.