Authors deserves respect for keeping this book free.
We were lucky to have a paperback low-cost-poorer-countries edition of The MINIX Book[1]. The code in the Minix book was an eye opener.The code clarifies the concept and sharpens the understanding.
Later on in my career as driver/firmware programmer, Computer Systems - A Programmers Perspective[2] and Unix Systems for Modern Architectures[3] helped a lot to clarify confusions and mysteries.