Go to the FAAs Advisory Circular Listing and down load:
AC 65-9A Airframe and Powerplant Mechanics General Handbook
AC 65-12A Airframe and Powerplant Mechanics Powerplant Handbook
AC 65-15A Airframe and Powerplant Mechanics Airframe Handbook
Each is a 200 or so page text book on aircraft maintenance. They were the test books I had to buy when I went to A&P school.
More accurately: you need an A&P license to sign off the work you do as a mechanic.
There are plently of very capable mechanics that do not have a rating. However, I want to emphasize, an A&P is very valuable to have as a mechanic. And in a perfect world every mechanic would have their ratings.