With the selection process for a command course the company usually has made sure you know the procedures and book in considerable depth. The course is mostly about the new responsibility (read the corresponding OM A chapters) and managing situations that are not as clear cut as the normal V1 cut. While it is good to keep your general knowledge up, i would focus rather on general leadership, command and CRM stuff. There are some fantastic books out there one can read and learn from. I learned quite a lot from books like „QF32“ by Richard de Crespigny, „Aircraft Command Techniques“ by Sal J. Falluco, „Controlling Pilot Error: Culture, Environment and CRM“ by Tony T. Kern and quite a few others, sadly most are in my other home 3 hours (by plane) away and i don’t remember all the exact titles. But I’m sure others can chip in with excellent recommendations on what to read. And it doesn’t hurt to continue reading after you got your command, we all can learn every day.