Could you tell me the best solution for a big loss of my computer ebook files?

I was thinking even to make a combined file of all the books made by every writer, writing on the file the name of the author, and ordering them by name.

Are they still in your recycle bin?
If not, if it was not too long ago, you can try to use an undelete program to restore the files.

You know, I didn’t think about that. THANK YOU VERY MUCH! :)

Just to ask: 12 days are enough?

I meant, if the files were deleted 12 days ago, is it enough time to recover them?

depends on how often you save and delete files.
when you “delete” a file, and if it is not moved to the “recycle bin”, it is not actually deleted, but its entry in the file allocation table is deleted, meaning the file still exists, the system just does not know where it is anymore.
the longer a file stays “deleted” like this, the more likely it becomes that the data belonging to it is overwritten by another file.

(I have my recycle bin set to NOT delete the files until/unless I empty it manually.)