Whatever I can find on sale mainly. But I do have a weakness for Maggie Stiefvater's books (the covers are gorgeous and the writing is a dream
) so I buy all of those in hardback so I can display them. I prefer paperback when I am carrying a book around in my bag though.
I'm the same with on sale books. I really love the site 'thriftbooks' because they have books for great prices and you can get free shipping on orders over $25. And secondhand book shops are like crack to me. I can never walk past one without buying at least a book.
I go either way, but I prefer hardback mainly... but on saying that, paperback is cheaper and lighter for portability.
But if cost, space, and storage were not an issue, my ideal would be leather-bound hardcover.
I have a few leather-bound books, both soft and hardcover and they are gorgeous. I'm also partial to really old books. My father gave me some from 1830 he found in a yard sale and they're the center of my 'collection' bookshelf.
Go dessadoll! I've gotten two planned out but haven't started crafting any of them