Hwang Kee is generally considered the father of TSD, his books are a place start.
Most of the forms that are practiced are from Shotokan and subsequently can be traced to Okinawa, a few are Chinese, and some Hwang Kee made himself (Chil Sung and Yuk Ro sets).
In his books Hwang Kee admits learning from a Karate book in Seoul Korea (probably by Gichin Funakoshi), watching Taekkyeon until he became proficient (not from receiving instruction), learning Kung Fu/Tai Chi while working in China, and reading the Muye Dobo Tongji. From his exposure to these sources he derived Tang Soo Do/Soo Bak Do. There is a lot of controversy with the origin of Tang Soo Do and a lot of crap about it being a pure 2,000 year old Korean art.
The Muye Dobo Tongji is an interesting read about Korean martial ways but not a history of Tang Soo Do. I recommend it on its own merit as an interesting martial arts history book.