Beware the Odd Angles
This counts as a Mythos book, although most information in it is unrelated to the Cthulhu Mythos. It adds +10% to Occult, +2% to Cthulhu Mythos, and costs 1D4 SAN to read.
Book by Francis Barret, first edition, 1801. This book helped start the occult renaissance of the 19th century. Francis Barret was considered a learned man by some and a fraud by others. His book has become one of the mainstays of modern occultism, and deals with alchemy, astrology, ceremonial magic, and deomonology.