I just started preaching through Ecclesiastes.
Here are two really good books on the subject.
Joy at the End of the Tether by Douglas Wilson
I use Wilson's outline in my introduction sermon as the structure of the book.
This book is worth reading over and over.
Ecclesiastes with New Eyes: A Table in the Mist by Jeffrey Meyers
Meyers' book is more in depth and yet very spiritually edifying (in contrast to dry commentary). Very helpful.