Principia Softwarica is a series of books explaining how things work in a computer by describing with full details all the source code of all the essential programs used by a programmer. All the programs come from Plan 9 from Bell Labs, an operating system designed as the successor to Unix whose code is remarkably elegant and small.

Not a “how-to”, but a “why-to” book about Unix expertise, that teaches you the things about Unix development that Unix experts know, but aren’t aware that they know.