Chapter 13 of the Airplane Flying Handbook, FAA-H-8083-3C, Transition to Multiengine Airplanes is a valuable resource for applicants seeking their multiengine ratings.
The FAA publication Flying Light Twins Safely is a succinct analysis of the factors to consider when flying light twin engine aircraft with one engine inoperative, including a discussion of the critical engine, minimum controllable airspeed, and Vmc demonstration.