The Scientist & Engineer's Guide to Digital Signal Processing
This is the hardcover edition of Smith's classic book.  It is also available in softcover under the title: "Digital Signal Processing: A Guide for Engineers and Scientists."  The softcover version is slightly less expensive, while the hardcover edition contains an additional 23 page study guide to help newcomers to DSP.  The Author's recommendation: Choose the hardcover edition if you will need DSP in the future, and the softcover version if your need is only short-term.