I like the idea of having all the rules I need under one cover. I also like the idea of the Libram Novus, wherein optional rules are presented - as long as those rules shouldn't already be in the main core rulebook.
What I mean is, if this is a fantasy roleplaying game, and the main rule book has Humans, Elves, Dwarves, but Elves in another supplement, this may be a bit awkward. It's not a cost/price thing, just a 'keep it tidy and in order' thing. Spells in particular could get out of hand very quickly.
Here's a good idea (even if I say so myself), you could present these optional Libram Novus rules in a style similar to Harnmaster. This way you can just print out and slot the loose leafed pages in your ever growing folder as and when you want. Naming convention for pages wouldn't be a 1, 2, 3 and so on, it would be sectioned. Like, Character 1, Character 2...Character 23. Then perhaps, Races 1, Races 2 etc.
Looking forward to this either way.