As gdl notes, your written English is very good: there's two non-idiomatic usages (was in "was living abroad" and texts in "writing english texts") and in your previous comment (
http://news.ycombinator.com/item?id=1683335) I simply can't tell your not an native English speaker (well, "landmark" is probably an incorrect usage since by definition it doesn't sound like you have been or can be very influential).
Elements of Style is the first book everyone suggests, although I didn't find it all that useful when I got around to buying it. I'd suggest checking it out and then later getting more prescriptive books.
While I'm not sure I'd recommend it as a second or even a fourth book, I'm rather fond of the rather old fashioned (OK, it's a century old...) and most certainly not American The King's English: http://en.wikipedia.org/wiki/The_King%27s_English
There's an on-line copy of the 2nd edition linked in the above that should give you an idea of if and when you want to try it; at the very least you should try this bit right now: http://www.bartleby.com/116/101.html (well, I love it...).
My copy is this recent printing of the 3rd edition: http://www.amazon.com/Kings-English-3rd-H-Fowler/dp/B0015NYN...