If you are used to reading grammar (meaning you can understand grammar concepts without the need of examples), then I would recommend Essential Spanish Grammar, published by Routledge. They have a series of these books for different languages and most of them are very good at presenting clear information. I can see that you are used to exposing yourself to different languages, so I don't think it would be a problem for you. I do not recommend it for people who study a language for the first time since the explanations tend to be very technical. There is also Comprehensive Spanish Grammar, basically the same, but a lot more details, exceptions, and so on. I would recommend the first one... once you speak Spanish fluently it will be a lot easier to read the second.

Gramática básica del estudiante de español. Difusión (A1-B1, tiene ejercicios, los dibujos que acompañan a las explicaciones son geniales) Gramática de uso del español. Teoría y práctica. SM ELE (hay varios niveles, tiene ejercicios) Gramática explicada para niveles intermedios. En clave ELE (A1-B2, tiene ejercicios) Gramática del español lengua extranjera. Edelsa. (no tiene ejercicios) All the above are books, I do not know any electronic or online grammars.

Thank you for your suggestion, I'll try it!

You can download a free unit from the second suggestion Marta made here: http://www.sm-ele.com/ver_noticia.aspx?id=25979 I also like that SM ELE series because they tend to be clear and there are exercises to practise ;)