Japanese Grammar!

The most recommended ones are Genki, Minna no Nihongo and Japanese from Zero, I believe.

If you want a free alternative, Tae-Kim’s grammar guide is the one I used mostly at the beginning, and it does a pretty decent job at teaching you the basics, though it’s a bit lacking in exercises.

If you are interested in reading manga in particular, Japanese the Manga Way is considered to be quite good too.

Here’s a comprehensive list of resource, including grammar and other aspects of japanese: