Thanks for the answers and thanks for the link provided by chaley.
I still have some problems with the author field, so I did this little experiment.
1. Deleted all books.
2. In menu "Preferences->Tweak->Author sort name algorithm" I put author_sort_copy_method = 'copy'
3. In menu "Preferences->Adding books" I check option "Read metadata from file contents rather than filename". The other two options are unchecked.
4. Close and open Calibre.
5. Add books just from one author, Isaac Asimov. As I said before, I edited all books with Sigil so the author field reads "Asimov, Isaac".
6. Now the Calibre browser shows some books as written by "Asimov, Isaac" and others by "Isaac Asimov". The author menu on the left shows there are two authors. Even in the Calibre library directory (I use Windows 7) there are two directories.
7. Then, I notice that all books listed as "Asimov, Isaac" have the label "language" set to English, while in the others the label is set to Spanish. Take in account that all these books are in Spanish.
8. Using Sigil, I changed the language label to Spanish.
9. Delete all books and add them again.
10. Now the Calibre browser shows all books as written by "Isaac Asimov", there is only one author, and one directory in the Calibre library. Even the "author sort" field is set to "Isaac Asimov".
Does this make sense? Not for me
What I'm doing wrong? I only want to see the books as written by "Asimov, Isaac" with "author sort" field set to "Asimov, Isaac" too.
*** Edit ***
I know I can edit authors by hand, but I don't want to. There are many books and authors and I already spend all the weekend editing the books and changing the "author" label to "last name, first name".
I did this because, as some may know, Spanish names are complex. Some people have last names with many words like "Calderon de la Barca". Some other people use two last names like "Arturo Perez Reverte" and the sort should be on the paternal last name (Perez in the example).