It should be Influence: the Psychology of Persuasion by Dr. Robert Cialdini. 
Put just one of the 6 marketing techniques he describes in this book to work and you can double your results.
He talks about:
- Reciprocation
- Commitment & Consistency
- Social Proof
- Liking
- Authority
- Scarcity
As you read the book you’ll recognize techniques that have been used on you by advertisers, your kids, teachers, employees, bosses, relatives, neighbors, fundraisers, and just about anyone who has ever wanted to get you to do something.
For example, in the chapter on scarcity, he explains why when you see an offer that says “limited to the first 100 people who reply” your instinct to respond is much stronger than it would be otherwise.
Even if you choose not to use these techniques in your own marketing, it’s very useful to understand why they work the way they do.
Note: I think Influence is out of print again, but the title and book cover in this post link back to Amazon, where you can usually get used copies.