Here are the two best selling books on preaching through this site.
Number two is the The Practice of Preaching by Paul Scott Wilson. This is one of my favorite books on preaching because it takes you through a typical sermon preparation week. You begin by looking at the text. Tons of questions are included to help you exegete the text. Then you go to looking at how you will put the sermon together. Once again there is a lot of aid here as well. The book is meant to be a textbook of preaching and thus has a lot of exercizes to help the beginning preacher. I cannot recommend the book enough and would suggest that it be a part of every beginning preachers library.
Finally, the best selling book through this website is They Like to Never Quit Praisin\’ God: The Role of Celebration in Preaching by Frank Thomas. This book is recommended highly by others including the Senior pastor of Trinity United Church of Christ Dr. Jeremiah Wright who said that if he had only three books to recommend it would be this one, Gardner Taylors book, and Holy Spirit and Preaching by James Forbes. This book takes you step by step in preaching a sermon that has Henry Mitchell’s concept of celebration at its center. This book revolutionized my own preaching and I would definitely suggest you purchase it if you can. You can also obtain a set of audio lessons based on the book from the author at this link.