Original Sources:
If you want to read more about the Arnold and Andre saga from those who were there, or spoke to those who were, check out these 19th century accounts
Written in 1834 by Jared Sparks, this account gives an almost minute-by-minute transpiring of the Andre/Arnold saga. Sparks interviewed those who participated in it, including Washington's spy chief Benj Tallmadge. It is very readable and free (out of copyright)
This 1899 account includes very early photos of the Andre/Arnold sites. William Abbatt wrote this as a quasi-history / tourist guide for visiting the sites. Also a very good read (and free!).
A classic war memoir by Washington's head of intelligence, and the first one to figure out that Arnold and Andre were up to something sinister. Tallmadge was also a cavalry office in the Revolutionary War, making it an engaging read.