Long Lining is a SERIOUS art... You and your horse can become injured and scared half to death if done incorrectly. I enjoy long lining, but I had a GREAT teacher help me with it early on. It is a dying art, not used (correctly) by very many current trainers. I would suggest trying to find one though, and getting some hands on help.
There are two books I found useful when continuing my long lining experience. The first is "From Birth to Backing". It's a book written by a short Englishman on everything from halter breaking a baby to the first few rides. It has a good section on long lining and has some trouble shooting tips at the back. Amazon.com: From Birth to Backing: Richard Maxwell, Johanna Sharples: Books
The next book is "Longeing and Long Lining" by Cherry Hill. This is a great book with a lot of information on both lunging and long lining, benefits and concerns of both, with good photos and illustrations. Amazon.com: Longeing and Long Lining, The English and Western Horse: A Total Program (Howell Reference Books): Cherry Hill: Books
She also has a book, "101 Longeing and Long Lining Exercises". Amazon.com: 101 Longeing and Longlining Exercises: Cherry Hill: Books
Books are great for reference or as a starting point, but I'd HIGHLY recommend getting some in-person help when you first start out.