Reddit reviews Practical Electronics Handbook

There are two main parts of "everything else": Mechanical, and Electronic.

For most hobby-level robotics, the mechanical side of things is pretty simple. Learn about torque and power, and how to change them around (gears, pulleys, levers, etc.) and you're most of the way there. As long as your robot is small and moves reasonably slowly, it doesn't get much harder than that. I can't really offer a good book that covers this because my major is Mechanical Engineering and so my knowledge comes from a wide variety of textbooks and lectures.

The electronic side is much more involved. The best thing that you can do is learn how to solve systems of equations. Learn Ohm's law. Learn Kirchhoff's current and voltage laws. Learn how to do loop current analysis.... The most important thing for a beginner in electronics is to realize that circuit schematics are not flow charts. Current does not flow down one wire and then another. Everything is moving all the time. I strongly recommend Practical Electronics Handbook by Sinclair and Dunton.

Once you get into the more advanced robots, when you have a real budget and you're doing more complex things, believe it or not, the electronic side gets to be the easy part. The faster your robot moves and the more complicated moves it can make, the more advanced your mechanical analysis needs to be. College-level textbooks on kinematics, statics, and dynamics are your best bet. And just like the mechanics, the programming gets complicated much faster than the electronics. If you want to make a robot that can drive or fly around quickly, you get pretty deep into nonlinear control theory. Same for robot arms and motion planning.

Oh, as far as recommendations for actual robot products go, check out sparkfun and adafruit and see if any kits really catch your attention. Otherwise, Mindstorms products make the mechanical and electronic assembly of the robot much easier - you don't have to worry about the electronics at all, and the mechanics are just lego technic. That said, you won't learn any electronic theory from playing with Mindstorms, so it's a bit of a compromise.