Hands-down, the BEST book about how Oracle's complex optimiser works. The title says "volume 1" and I met Lewis at Oracle OpenWorld a few years back and he confirmed my suspicions that writing this book was so hard, he was too exhausted to write a sequel. It is difficult but an excellent source of information. When you realize that Oracle has as many patents and programmers for their optimiser as Microsoft has for their Windows operating system, you get an idea of the titanic work it required to write this extraordinary book!
Nobody (including those who programmed it themselves) understands the Oracle CBO better than Jonathan Lewis. If you want to make sense out of how and why Oracle chooses to execute queries in the way that it does, read this book. It is dense, complicated material but Lewis nevertheless does his utmost to make it both clear and comprehensive.
After asking myself why some of the data models were performing so badly I realised I had to read this book (sure, among others). But this was the first book about cost-based optimisation I was able to read from cover to cover... and understand how to use the myriad of features Oracle databases have to offer. The authors know CBO extremely well and the book is written in a way that a good developer (doesn't need to be a seasoned or expert) will take huge advantage of.