There are really only two decent books on wholesaling I know of - one by Todd Fleming called something like If you can't wholesale after this, I've got nothing for you, and the BP book by Jamil Damji. Those are good resources, but what you will find is that most of the closing is learned in the field by connecting with people.
A couple of tips:
1. Learn repair costs asap. Befriend contactors and other investors at meetups so you can build a catalog of how much everything costs. Most wholesalers just make this up and so the deals are worthless.
2. Focus on the person (the seller), not the offer. The person has to go somewhere. The person has a problem that needs solving. If you can solve the problem, you can get the house for less than everyone else who doesn't care about the person.
3. Don't oversell the ARV. Classic disaster which just tells us you bought too high.
4. Don't say you have cash, if you don't have cash. If you want seller finance, learn how to explain it well.
Good luck.