Can anyone recommend me to any great books on grammar? Please don't recommend Strunk and White (I have it already)
The Little, Brown Handbook. The Scott, Foresman Handbook for Writers. And for answering the really nitpicky questions, The Chicago Manual of Style. The first through can be read cover to cover. The CMS is not very reader friendly, but if you're looking for a definitive answer, it's the place to look. All three are priimarily for US English. The Penguin books have a good reputation for UK English.
I also highly recommend The Little, Brown Handbook. It's currently in its 11th edition and a new hard-bound copy will cost you some serious coin, about $60 or so, but is well worth it. If money is an issue, look for a used copy. I have the 4th edition, published in 1989, and can't see any reason to buy a newer edition.
I want to echo this. Penguin is - I very much believe - a writer's best friend. If you are really wishing to improve your grammar the thing I suggest above all else is to read widely and read a lot. The Penguin Classics are in, my experience and opinion, some of the best books available; very good printing, with good notes and introductions.
The Oxford English Grammar, by Sidney Greenbaum is huge and intimidating, but authoritative. And somebody should mention Fowler's Modern English Usage, because it's so darn classic. So I just did.
Almost certainly the best - the first couple of editions anyhow - but not necessarily the most useful.
Actually I did think of something other than Strunk and White... The Transitive Vampire. It's silly but a lot of fun and can be very useful.
"My Grammar and I (Or Should That Be 'Me'?): Old-School Ways to Sharpen Your English by Caroline Taggart and J A Wines is an excellent and enjoyable read. It left me wondering how anyone ever succeeds in learning English as a second language.