1.) You must know the grammar of Spanish (your mother tongue). If you have not studied it, do that first before learning the grammar of a foreign language. 2.) The best book is one that is available to you in your local librería o biblioteca. But If you can order by mail then... 3.) Murphy's for a grammar written in inglés, or "Gramática De La Lengua Inglesa"/Taylor en español.

Thanks Victoria

Murphy' s grammar is the best and easy explained. :)