I've only recently started my studying for the HVAC portion but I have used the books at work. I'm almost through my first pass on the Six Minute Solutions HVAC book, got about 10 questions left. I know a lot of people on here have mixed feelings for this book, especially the TFS guys, because they say a lot of the questions aren't reasonable or are way out of left field. I haven't done the NCEES practice tests yet so I can't confirm this. But as I've done the SMS book, I think I've picked out the problems that people would consider "out of left field." These are problems you won't find solutions or equations to in the MERM. So far, ALL of these "weird" problems use an equation or an excerpt of information from one of the ASHRAE books.
So my advise right now would be to work the SMS book and those particular problems will force you to start looking things up in these books and it'll help you get a feel of whats in there and where to look. If you're stuck, use the index, it's one combined index for all 4 books in the back of each (at least that's how it is in the latest editions).
Generally start with the green Fundamentals book first. It'll have anything you need for load calculation problems, tricky window and fenestration problems, and lot of of good info on the basic HVAC topics that might be considered obscure on the test. The Applications book will be useful if you encounter a problem dealing with a specific application of an HVAC system. For example, there's a whole section in there about how to design systems for indoor pools. There was a question in the SMS about that. Use the Systems and Equipment book for questions that would ask you something specifically about the characteristics of a particular HVAC system. Like what type of chiller would be used for a XXXX ton load, etc... I haven't had to crack the Refrigeration book yet, but I've only just started my long list of practice problems so maybe I will down the road.
