You must be on v1.7.2 still. Very sensible, I suggest you stay with it.
The file is still there on the hard drive.
If you look at the 'item info' (either using the selectable option you mentioned, or selecting a book and using ctrl-I) it will tell you where on the hard drive it lives.
All the books are likely to be in the savme directory, so if that book is deleted from ADE, check a different book instead.
Probably in 'My Digital Editions' directory under Documents; where Documents is will vary with different Windows versions.