Can you recommend me a sales book/resources

So a bit of a back story, until now I've relied purely on instinct/natural charm to make offline sales, mainly for SEO packages to small and medium sized businesses. Most of my leads come through word of mouth and referrals which are pretty warm leads by anyone's standards, so it hasn't particularly been difficult to convert them into sales.

Lately I've been looking to add a non SEO revenue stream to my businesses and have settled on 7878's reputation management system as there's great opportunities for up-sells for my existing business and many of my current clients should be easy initial sales. I may even start a journey thread on it (I know, another one!) down the line.

My thoughts are, if I'm going to the trouble to mail out letters using a pretty polished system, I don't want my sales skills to let me down when I get a meeting/phone call from it. I know there are some pretty sharp guys/marketers on BHW, so I'm wondering people would suggest some books/resources to sharpen up my sales skills, that people have personally had success with.

Thanks.

you can try lynda courses
some of them are very good

CA$HVERTISING by Drew Eric Whitman - essential, especially if you're going to do direct mail
Influence by Robert B. Cialdini - great book about consumer psychology, covering interesting topics such as the law of reciprocity. Even the author above mentions it in his book.