Adding David Udell and Polyphony Bruna as co-authors. A new chapter on mechanistic interpretability, and extensive revisions to the transformer chapter, including an expanded discussion of cognitive science and discussion of the residual stream. A new section on the geometry of word embeddings (e.g. the famous king-queen example). Folding in updates to the linear algebra chapter on vector comparison methods and matrix products. Another pass on most of this material should be coming later in the summer.