The Plain English Guide by Martin Cutts
This guide is for anyone who writes public information leaflets, letters and reports. It has a very useful chapter on layout problems and solutions. It contains everything you ever wanted to know about plain language, word choice, voice, legal language.
The Complete Plain Words by Sir Ernest Gower
In the author's words:
"The main purpose of this book is to help officials in their use of written English as a tool of their trade."
It needs no further recommendation!
Troublesome Words by Bill Bryson
An extremely entertaining and informative book. Even if you are not at all interested in English language, you are sure to find this book is fun to read.