Amazon has the Look Inside the Book feature that you can check out. For their NOOK, Barnes and Noble will download you a sample from the book so you can decide if you want to buy it. I know lately, I've sent both of those companies complaint emails. All I got from the "sample" chapters was the copyright page, table of contents, and the introduction. For one book, I kid you not, I got exactly one page of the first chapter of the book. The rest was all that useless filler stuff. How are you going to decide if you want to buy an ebook if they don't give you enough of the book to make a decision?
And fyi, you don't have to have one of those devices to get ebooks. Both Kindle and NOOK have apps that you can download so you can read their books on your pc.