Learning from the Singer 1997 paper, I was able to run hierarchical models by Proc Mixed easily. However, reading "SAS System for Mixed Models" got me more confused about what I was able to utilize easily. Not to mention, I spent more than $90 back then. Don't waste your money on it.
Wonderful Reference
Rating: 5/5
This volume, while focusing on the Mixed procedure in SAS, gives a great overview of mixed models theory and application. Full of examples from a variety of fields and better yet, real data to hone one's skills.
Wonderful resource
Rating: 5/5
Tons of examples! It's a nice mix between theory and applications, citing scenarios from all industries. Not only do they show you the code, they also produce the output and go through it section by section.