"Putting To A Pair" is an essential starter guide, then "Carriage Driving" by Heike Bean and of course anything by Sallie Walrond is good. Sorry to be so brief but I'm afraid my power is about to be knocked out by this windstorm!
The above mentioned books are great. We have both. If you are thinking of doing draft hitch team, another is "A Teamster's View-More and Different" by Steve Bowers.
OOOO - Performancemini - I hadn't seen that one yet!!!
Whether doing a draft team or a regular driving team there is a HUGE wealth of info in the 2 books by Lynn Miller. They are based on driving draft teams and some (not much) terminology is different - but there are pointers on how to build your own equipment, fitting harness, training, awesome descriptions with pictures of handling lines of multiple horses etc. Also, some great pictures of horses AND shetlands working in the yesteryear time frame as well as currently.