The MERM, all 4 ASHRAE books, and the Six Minute Solutions (HVAC) are all you really need in addition to the sample exams. The April exam, IMO, had more complex problems then you will run into within these practice problems, but they do a good job of providing a basis for understanding and applying the material/concepts.
If I have to take the exam again, I will be focusing most of my effort on the Fluids, Power Cycles, and HVAC topics in the MERM and and to a lesser extent, Thermodynamics and Heat Transfer. Also, be prepared for the MD questions in the morning exam. The fluids and HVAC are easy enough since you need to study those topics for the afternoon, but there were quite a few MD questions which was my weakest subject.