Reference Registry

External references normalized in one place instead of repeated across every branch index.

Books (canonical)

Database Internals — Alex Petrov. Storage engines, B-Trees, WAL, and transactions — the conceptual spine of the database-engine branches.
Designing Data-Intensive Applications — Martin Kleppmann. Storage and retrieval, replication, and transactions, ch. 3 especially.
Advanced Programming in the UNIX Environment — Stevens & Rago. Process control, signals, and job control for the shell branch.
The Linux Programming Interface — Michael Kerrisk. The canonical syscall reference, used throughout.
Test-Driven Development for Embedded C — James Grenning. TDD discipline for systems code, carried over from Atlas I.
The Art of Multiprocessor Programming — Herlihy & Shavit. Lock-free data structures, for concurrency-in-practice.

RFC 7230 / RFC 7231 — HTTP/1.1, the normative spec behind the HTTP server.
SQLite file format spec and "Architecture of SQLite" (sqlite.org/arch.html) — a real-world reference implementation for the database engine.
man epoll, man kqueue — read past the tutorials.

codecrafters.io — "Build Your Own Redis/Git/SQLite/HTTP-Server/Shell" — staged, testable milestones for most of these branches.
build-your-own.org — the Go database-in-a-book that this atlas's database-engine progression mirrors (in spirit; this atlas is C-first).
swtch.com/~rsc/regexp/ — Russ Cox's regex-engine series, the spine of the regex-engine branch.
wrk / hey — HTTP load-testing tools used for every benchmark in concurrency-in-practice.
libFuzzer / AFL++ — binary-format and parser fuzzing, carried over from Atlas I's craftsmanship branch.