English

I learnt proper English some years ago. Strict, in the grammar sense, English. However, more and more, I see my abilities being deteriorated due to the internet. Even native speakers make constant mistakes, being some horrendous, which from those, unfortunately, I am beginning to be a victim, as I can notice them in my own speaking.

Would someone mid recommending a good book I can download from which I can revise some concepts?
Also, is there any online resource for prepositions (these I always had trouble with)?

Thank you, /lang/

Are you guys serious?