There are uncounted books on Yang Long Style Tai Chi Chaun. Unfortunately it is impossible to really learn from them, instead they add more texture to ones studies.
I'd suggest check out your local Barnes and Nobels or Borders bookstore in the Sports Section. You should find a wide range of books.
Try and find something similar to the version you studied (there are literally hundreds of versions of the Long Form).
The better books might have decades worth of material, but unless you study for decades yourself they won't make a great deal of sense.
Among the possibilites, the books of Yang Jwing-Ming. But don't be surprised if the books, while similar, are also different.