Sealoc and Calder
Haven't seen Calder's Marine Diesel but his "Boat Owners Mechanical and Electrical Manual" contains some sections on how diesels work that are probably similar and are just excellent for beginners. Calder starts at the beginning assuming you know nothing (was a fair assumption with me) and explains how things work and how to maintain and fix them. He goes in far more detail than you'll probably need. the Mechanical and electrical manual also provides an explanation of how the other major systems (AC, DC, plumbing, head, stearing, rigging, etc.) on your boat work, can be maintained and fixed. Sealoc makes a manual for Yanmar's that is like Chilton's for autos. Has lots of pictures and step by step instructions on how to do things like change oil and bleeding. Excellent for begginers. You can get is on-line or WM or Boat US for about $25.The official Yanmar manual is very good, but a little cryptic if you have no idea what you are doing.