Gramática Moderna de la Lengua Japonesa

Just about the best Japanese grammar book if you are fluent in Spanish. The only bad thing about it is, it expects learners to delay (or skip altogether) learning the kanas and kanji, so it uses romanized text instead, in a version adapted for Spanish speakers to top it off. This may sound appealing to beginners and lazy students, but once you get up to speed with real written Japanese it becomes a nuisance.