In Book 1, you built binary search trees (Ch 25) and saw why B-trees minimise disk reads (Ch 26). The core insight of Ch 26: a disk read has a fixed cost regardless of how much data you read within a page boundary. A B-tree node is sized to match a disk page, so each level of the tree costs exactly one disk read.
This chapter traces Thread 2 (Trees) into three production systems. In each case, the specific tree variant is determined by the same analysis: what are the constraints of the storage medium, and what tree structure matches those constraints? The thread continues in Book 3, Ch 9, where B-tree indexes become the foundation of query optimisation.