Did the same thing.
Acquired the gleim for my FAA IR and just learned all the answers so I could get the written test out of the way immediatly. Took me a week but afterwards my instructor and I went through all the bits and in depth material in order to pass the oral part of the IR.
By all means get the gleim so you can pass the written test but don't expect to pass the oral with it. The PTS manual is okay but you really need to practice with other students/instructors in order to properly understand everything. Everything is basically: How to read the approach plate, what does everything on it mean? Maltese cross is what? precision approach minima is? Is it viable as an alternate i.e. does it say that you can use it as an alternate? 1-1-2-3 and S.I.A.P. what does it mean? What do you do if you have a radio failure?
Know your aircraft, it's speeds, tire pressures, fuel is supplied how? Can you draw the fuel system? What is the six pack, explain to me all 6. What does this weather chart say? how long is it valid for? Where on this chart is D airspace? Show me a C.O.P. on this chart, what is it? Where can you start to descend/climb on this airway (knocked me off that did!).
I could go on forever but I'll end it here.