Thank you, ZapperZ.
With the disclaimer that I myself have no plans to work my way through any of the famous math textbooks, I'll make the following observation. It would be nice to have sections devoted to specific questions about specific textbooks (as opposed to discussions about the general merits of the book). Periodically, one sees threads on math forums with the theme "Anybody interested in forming a group to work our way through [famous book name]?". I don't recall examples where such groups materialized - at least within math forums. A section devoted to a specific book wouldn't exactly be a collaborative group, but I think it would be the best practical imitation of such a group. (Perhaps other science forums have sections devoted to particular books. Do they?)
One can argue that any specific question about material in a textbook can be answered from a broader perspective of general knowledge about the subject and that this broader perspective is the "best" answer. (Nothing practical can be decided without considering Philosophy and that's one philosophical point of view.) But just from the point of view of having a popular forum, I speculate that sections for discussing the specifics of particular books would be useful. (I don't mean than a book section should be created on every request by users. The moderators will know the books that are most often mentioned.)
I don't know how forum software works, but I imagine that creating sections for each particular book would be an administrative headache. So perhaps the place to start is in requesting features that would make the task easier from the company that writes the forum software.