What are the best books for helping sales-people develop positive thinking skills and deal with rejection?

I don’t particularly need books on sales techniques. I mostly need books on personal psychology techniques. The individual I’m trying to help is the top salesperson in the company, but she gets discouraged much too easily whenever she has a period of time with few or no sales. She tends to allow that discouragement and pessimism to affect her performance, thus perpetuating the less successful period.

"How to win friens and influence people" by Dale Carnegie