You'll be learning about prop governors, constant speed props, manifold pressure, and retractible gear. Those are the topics you should investigate.
As SkyGuyEd indicated AvWeb has four terrific articles worth reading. "Mixture Magic", "Manifold Pressure Sucks", "Those Marvelous Props", and "Putting It All Together". You should definitely check them out.
A systems book would also be helpful. The Tab Practical Flying Series has a good beginner's book entitled "Aircraft Systems" by David Lombardo.
Rod Machado's Private Pilot Manual also does a decent job of introducing and discussing props and MP.
You'll probably have a tougher time finding information on gear because you basically lower it and raise it. However, during your training you'll find, and should look for, information on speeds at which the gear can be raised, lowered, and flown with the gear down (those might be the same speed or three separate speeds, btw). You should look for information on *how* the gear is extended and retracted, whether the nose gear retracts forward or rearward, and what kind of preflight is involved. You'll also want to look for information on how the emergency gear extension works. There are basically three methods - free fall, hand pumped, and gas assisted.
The endorsement itself is not difficult. There's no checkride involved. Some ground training and a flight or two, and you'll have it in your logbook. However, the endorsement gives you access to more complex aircraft and as such you should find out all you can about more complex systems. It opens up a whole new world of additional study opportunity!
Best of luck on the instrument ride, btw.