http://www.merlotx.org/ is the main site for the online textbooks of all different categories. The link above is for Literature but the merlotx.org site covers just about every subject. Merlot itself is a great resource but this subsection can be very useful.
Diigo Group lists built from scratch by subject experts could yield low cost-alternatives to old-fashioned texts as suggested in this Wired Campus article.
Flat World Knowledge: "We are the world's largest publisher of free and open college textbooks. With our ever-expanding catalog of top quality books by expert authors, now is your chance to be a hero and help your students save thousands of dollars. Get started today and join the textbook affordability movement."
Or classroom. Digital Flipbooks like "Flipsnack" are an interesting way to create online content that connects learners with innovation. Through virtually flipping pages you can add .pdf's, url's, meme's, gifs, and other digital content to be stored in one virtual textbook. This would be great for manuals, textbooks, or any type of file sharing.
What a great way to customize the reading assignments and resources for students in our online courses. This seems like it could also save money on textbook costs for our students.
By KATIE DAWSON, Indiana Daily Student The Indiana University eText initiative has saved 5,300 IU students as much as $100,000 by allowing professors to select eTexts instead of traditional textbooks to teach in class. eTexts first became available for faculty to consider for classes in September 2011 but are now available for all classes to use this spring semester.
The Math Club at Virginia Commonwealth University provides this guide for the web's best math websites, math blogs, math YouTube videos, classic math textbooks, etc.
I am moving toward more tech and eventually considering textbook-free classes. Step one is trying a semester with Aplia, a text-linked software program that will allow me to entirely design the course using the online version of the text I selected for my students, complete with a grade book and assignment records for each student. I am excited to start using such a technology-intensive approach for my classes.