Alright, I've got to raise some questions here regarding using CRCs and/or identifiers.
If the CRC is based on the epub file, and the epub file included bookmark information, than the CRC will never be valid. Because every time a bookmark is added, removed or changed, the CRC will be altered.
Identifiers pose a similar problem because it implies that books never change. And the problem is that ebooks are changing. Ebooks are starting to have versions, just like software, due to corrections in page order, typos, etc. However, in most instances no version information or release note is being offered. Changes are being implemented without warning or anyone's knowledge. So effectively two people could have the exact same book. Except, they're different. Until some kind of major snafu occurs with a mis-quotation, things will be awkward. (Which is why I'm really rooting for the widespread appeal of e-textbooks and other non-fiction writings which need clear and accurate citations.)
Because of both of these issues, I would prefer that the user be asked where they want to save a file. Barring that, perhaps a user setting with a default Save To... path? At least then the user knows where to find any downloaded files.