Methods and Materials of Painting of the Great Schools and Masters by Sir Charles Lock Eastlake
I use this book as a reference when I want to know a little bit more about a specific material, technique, or artist… it is sort of like an encyclopedia for artists.
This book is especially helpful if I am interested in learning the classical methods employed by the masters. I will read specific section of this book depending on what I am researching. It has come in handy when I am trying to solve a problem with the different copies I am working on at the NGA.