Orison Swett Marden 
Selling Things [PDF ebook] 

This book will feel dated to the modern salesperson however, the tenets presented on how to successfully sell are as true today as they ever were and unfettered by modern references such as email and cell phones. A great ’back to the basics’ book for all of us in the sales profession.
Although written 100+ years ago, his style is not dated and hard to read, a bit formal if anything. You may not recognize the people whose stories he uses to illustrate his points, but the message is as clear as crystal even without this recognition.