There is no one best book. Some people like 1 resource while others prefer a different resource.
This topic has been beat to death and this question is always asked so not to many members will respond to this post. Your best option is to use the search function to bring up past discussions.
Actually, the best way to prepare for the NREMT CBT is to use a test prep service that will track deficiencies and help you study weaker areas. There are several of these services, JB Navigate is frequently listed as one of the best.