This repository contains an open-source textbook on metaprogramming in Lean 4, a formal proof assistant language. The book is authored by Arthur Paulino, Damiano Testa, Edward Ayers, Evgenia Karunus, Henrik Böving, Jannis Limperg, Siddhartha Gadgil, and Siddharth Bhat, and is made available in both HTML and PDF formats through the Lean Prover Community. The source material is written in Lean files and automatically converted to markdown using the mdgen tool, with contributors expected to edit the original Lean source code rather than the generated markdown files.