"After the Prophet" by Lesley Hazelton

"After The Prophet: The Epic Story Of The Shia Sunni Split In Islam" by Lesley Hazelton is one of the finest books written on the topic of shia-sunni problem in Islam. It is based on a neutral narrative and both the shia and sunni views about the incidents are given equal preference by the author in describing the incidents that took place after the death of Prophet Muhammad.
The book can be downloaded in PDF format from here.

Highly recommended for reading to those people who want to understand the basis of conflicts between Sunnis and Shias.