Add all normal and skill books to a list, add that list to every misc. trader in Skyrim.
Each trader has a different affinity for books.
One is guaranteed.

Modifies the following Merchant Chests (and is thus probably incompatible with anything else that modifies these Merchant Chests):

Riverwood Trader (Riverwood)
Belethor's Goods (Whiterun)
Arnleif and Sons (Markarth)
Pawned Prawn (Riften)
Brand-Shei (Riften)
Sadri's Used Wares (Windhelm)
Gray Pine Goods (Falthreath)
Bits and Pieces (Solitude)
East Empire Company (Solitude)
Urag gro-Shub (Winterhold - College)
Sorine Jurard (Dawnguard)
Ronthil (Castle Volkihar)
Fethis Alor (Raven Rock)
Khajiit Caravans (A, B, and C chests)