Books on Sales Enablement

 Hey all,

I am a product marketing manager supporting a team of 7 sales people who are distributed and spend most of their time in the field.  

One of  my goals this year is to boost the sales enablement function of my job by providing more effective tools and messaging to our sales people.  

I was wondering if anyone had any tips for good books to read on boosting the effectiveness of sales enablement.

Thanks in advance,

James