Reading the manual from beginning to end would be a nightmare. You are going about it the right way. Chances are, you will never need a good percentage of the features in reaper, so there is no point wasting time and brain space reading about them.
I think most people in your position will program the beats in midi and use a vst for the sounds. It really isn't that hard. You know what you want it to sound like, so you just have to count it out. Once you program a measure of the beat, just copy/paste it down the timeline. Then go back and replace it with fills where you want them.
Midi is just control information. ie. This note is played at this time for this long this hard. If you record the midi information, you have recorded the performance, but not the sound. Now, you can take that performance and apply it to any sound you wish.