The Elements of Style -Strunk and White's book is probably the most widely cited books on elegant writing. It is short and pithy, living up to its own rule 'omit needless words'. It covers American usage which is less bothersome for us Brits than you'd think, in part because American non-fiction prose shares a directness and precision with the best American literature.
The English, trying to be clever, often over-egg the pudding and this book is a good recipe against that. It has two other advantages. It is short and it is pocketsize - a good choice for a train journey.