I like to read books about marketing, they help me better understand clients and their needs.
I have one brilliant book on marketing called "401 Killer Marketing Tactics to Maximize Profits,
Increase Sales and Stomp Your Competition" by Tom Feltenstein. It has huge amount of practical ideas regarding the enhancement of your business. I think it is one of the best books on marketing.
The most interesting information for me is what the author of this book thinks about logo design:
"To be effective, it must be readable, clear, and bold."
"Use your logo whenerever possible-on matchbook covers, communication boards, direct mail, letterhead,
shopping bags, and so on."
Very good advice which in my humble opinion should be used by every logo designer.