I am taking a prep class through my job. We are using the PMBOK guide and also a study guide by Rita Mulcachy. My question is can I just use the Rita study guide to practice for the exam?

Studying from the PMBOK guide along with Rita’s book is sufficient as long as you really understand the concepts and cement the concepts learned by solving mock exams and/or sample questions from each process group / knowledge area. If you search thorougly, you will find a lot of free sample questions and mock exams on the Web. Practice them in order to increase your confidence, and then sit for the exam.

Best Regards,