there have been many discussions about classical guitar methods here in the forum.
Please have a look at this subforum
and study some of the threads there.
In a few words: there are different views about the "best" method or there are many good methods.
Sagreras is a good one, but depending on your background knowledge you might need additional books.
But more important than all methods is a good teacher to give you feedback and lead you on the way.