A good way to link all of the resources related to free online textbooks is a category. I did a quick search in the category namespace, using the options at the bottom of a regular seach page. It took me a few tries before I found something relevant: "open," "textbook," "free," aha, Category:Textbooks.
There's not much included. There may well be many other pages that could be added. I wonder if it'd be better to have a more specific category: Free online textbooks.