NCBI Handbook
"Although each NCBI resource has online help documentation associated with it, there is no cohesive approach to describing the databases and search engines, nor any significant information on how the databases work or how they can be leveraged, for bioinformatics research on a larger scale. The NCBI Handbook is designed to address this information gap.
Each chapter is devoted to one service; after a brief overview on using the resource, there is an account of how the resource works, including topics such as how data are included in a database, database design, query processing, and how the different resources relate to each other. "