Assuming you're talking about B2B, the best sales book I ever read is hands down Jill Conrath's Selling to Big Companies.
You can get your motivational guys like the little Red Book of Sales, etc....but really if you need to know how to sell read Conrath, even if your clients are big companies.
Book Yourself Solid by Michael Port is a great book for developing a personal brand and marketing a service.
2nd Book Yourself Solid.
Jeffrey Gitomer's little black book of connections : 6.5 assets for networking your way to rich relationships.
Duct tape marketing : the world's most practical small business marketing guide
The referral engine : teaching your business how to market itself
Also, lot's of good blogs by these and other authors.
How I Raised Myself from Failure to Success in Selling - Frank Bettger
http://www.amazon.com/Raised-Myself-Failure-Success-Selling/dp/067179437X "The most helpful and inspiring book on salsemanship that I have ever read."
- Dale Carnegie
My favorite as well.