Hi Luisa
The book is an resource, that correct! Check if the file book.php exist in your folder wwwrrot/moodledata/es_utf8. If not exist, you must copy the file book.php in this folder in the utf_8 format!
I check the file Luisa, my spanish begins and ends with ole ... so i have a spanish language file, but only a very old one. So open the book.php from the attachment and translate it from english to spanish.
Be carefull, here is an example:
$string['modulename'] = 'Book';
If u will translate this one line, this one word, you must copy it between the quotes ' ' and at the end of a line must be a ; so dont delete or overwrite this!!!
If u have problems to do that, write a message, i will translate it with your help to spanish.
Bye Andy