As Kovid says, Strunk & White's
Elements of Style is right up there as a first-y for grammar.
Following that, and with slightly more entertainment value, Lynne Truss's
Eats, Shoots and Leaves is a good read, and very popular. I enjoyed it, though her attitude sometimes left me thinking she couldn't see the substance of the forest for the style of the "leaves". I'd still recommend it though.
Don Watson's
Death Sentence and
Weasel Words are great for showing the "decay of language", driven from corporate doublespeak into political doublespeak. The former book is a cogent argument and advisory and a wonderful read, and the second more of a good reference. I recommend at least the former in conjunction with Lynn Truss's book, as it is referring to the "substance" of communication that I find is missing in Lynn's quest for the correctly placed apostrophe and comma. I think it (and
Weasel Words if you're so inclined) will serve you well in both speaking and writing.
The above four are my current sum of such texts. I look forward to other recommendations.
Cheers,
Marc