Thank you for answering.
Now, after rebuilding the books.db, recreating manually all the collections and adding several books, all seemed OK, but then, suddenly, I got a new error and calibre can't communicate with the device:
Cannot concatenate 'str' and 'NoneType' objects
Traceback (most recent call last):
File "site-packages\calibre\gui2\device.py", line 85, in run
File "site-packages\calibre\gui2\device.py", line 470, in _books
File "site-packages\calibre\devices\prst1\driver.py", line 197, in books
TypeError: cannot concatenate 'str' and 'NoneType' objects
That's a nightmare.
Can I fix that now without deleting again the books.db?
I think I should look for a different way to manage my collections... But that's a pity, because Calibre is the best, if I could fix these weird problems.