Such a project could pave the way for Open Access alternatives to "monolithic" textbooks.

Apart from cost, mainstream textbooks are often problematic because they are written by authors whose areas of specialization aren't coextensive with the textbook's material. So, some chapters ("learning modules" as A.T. appropriately puts it) received less attention than others.
Modular design would also make learning material easier to update. Instead of changing the whole "book," you can change one section at a time.
This does go well with Rice's CNX but it might be useful to think about the links between course material and learning strategies.