You can find a study guide for the Professional NABCEP exam http://www.nabcep.org/resources to be able to sit for the professional exam then you must have installed 1kW of solar electric systems and have 1 year of experience.
The exam is offered twice a year. This exam is harder to understand and is more difficult. It is set up for someone running a solar business. It includes OSHA regulations and standards. This is a voluntary credential.
On information on the difference between Certified PV Installer vs. The Entry Level, see the link.