An intriguing concept - bringing together free/open content/sources related to specific topics/areas of study.
It is, however, somewhat limited in its scope at this point, as well as being US-focused, eg. the History "book" is US History-only.
I looked in via the "Self Learner" section, as the Student and Educator options were asking for institution name plus the details of the book/text being sought. Not having a text in mind for the limited subjects offered, I thought the self learner option, which only requires you pick a subject, would give me some idea of what was on offer.
But have a go, and see what you think - there is such a push to move to a digital framework for our curricula and resources - maybe this is what you are looking for?