I've tried several different methods of bags/boards/tape and have finally found what I think is the perfect solution.
Mylar bags with clear backing boards and Scotch double sided tape.
After placing the book and board inside the bag I put a piece of the double sided tape on the back of the bag first.
Then I fold over the flap and press down over the tape. This does a couple of things that I like. Mylar makes my books look great and with the clear boards I can see the back cover too. The Scotch double sided tape almost completely disappears so as not to block the view of the back cover. Also, when applied as described above, the tape stays on the back of the bag and the flap opens with no tape attached, No worries of accidentally catching the tape on the book.