Authors do not always take the time to make their site user-friendly. If a site is hard to use, it will not be used and nor bookmarked. Page is simply a list of links with no apparent organization, many links only lead to other links instead of actual information. Web publishing easy with editors and composers. It is easier to put something on the Web than to maintain it.
Nothing is permanent on the Internet.
Page is an advertising site posted by a company dedicated more to online shopping than to providing stable links. Online books displayed change according to the whim of the advertizers. Collection is sponsored by a major university dedicated to creating and maintaining service.