Three free textbooks that actually teach, not link-farm

What it does

The Engineering Handbook is three complete books in plain Markdown: an HLD handbook (networking through distributed systems theory, architecture, reliability, security, and AI system design), a DSA handbook organized by problem pattern with LeetCode solutions in Python (most also in Java, C++, and Go), and an AI Engineering handbook on shipping production features on top of model APIs. Everything renders on GitHub, including the Mermaid diagrams, and the same content powers free websites with no sign-in.

The interesting bit

The stated rule is “no outlines, link lists, or ‘read this elsewhere’” — every chapter teaches the topic on the page, from first principles through production gotchas, with footnoted primary sources (papers, RFCs, postmortems). Chapters are also opinionated: they recommend an approach and spell out the trade-offs when reasonable people disagree. Each chapter records when it was last updated, which is rare discipline for free content.

Key highlights

• Three full curricula: HLD (12 parts, prerequisites to interview frameworks), DSA (pattern-based), and AI Engineering (prompts, evals, RAG, agents, observability, cost)
• Covers a genuinely current stack: LLM serving, MCP, CRDTs, edge computing, FinOps, post-quantum cryptography
• Trade-offs Library for design decisions; pattern decision pages for algorithm choices
• Study plans ranging from one weekend to the full curriculum
• CC BY-SA 4.0 — free to read, share, adapt, and build courses on; PRs and review welcome

Caveats

• Not for people who’ve never programmed — the README itself points beginners to CS50 or The Odin Project first
• Concepts over syntax: it won’t teach you any one language’s idioms

Verdict

Worth a bookmark if you’re prepping for system design or coding interviews, or moving into AI engineering without a formal CS background. If you wanted language-specific tutorials or a quick cheatsheet, this is the wrong shelf.