These little letter books have been really satisfying little projects. I almost always use scrap pieces to make them. The cover is usually a card stock and the inside pages are regular weight printer paper. Although, I do try to coordinate the inside pages with the feel of the outside. To accomplish this, I might use cream colored paper instead of white or embellish the edges of the interior papers with coordinating stamped images.
I have changed my binding technique based on one I learned from Esther K. Smith's book How to Make Books: Fold, Cut, & Stitch Your Way to a One-of-Kind Book. Now, I am binding my letter book like a signature you would find in a book. It is completely stitched top to bottom and has a much more finished look. I am very excited about this improvement.
If you aren't inspired to make one of your own, I am not doing my job properly. I should get back to work and come up with some better samples.