A book for storing masteries would only be useful if they actually make the things combinable to create better ones. As it is, even level IIs are so common that they are dropped on the ground (most level Is never leave the spawn site, and the IIs and a the remaining Is end up on the ground around moongates or in Luna), at least on my shard. Without at least a promise of a future ability to scrollbind them, keeping all those level I & II masteries is a fool's errand. I've done hundreds of champs that drop masteries, and have gotten maybe 5-6 level IIIs (only 1 of which I needed), and the other other III I've used, I was given.
A book for books, though? That's being silly. I was surprised that they didn't stick another book into the paperdoll for masteries (or alter the one that is for special moves), and went with a physical book. If you're talking about a bag to put spellbooks in, that allows the books to function without cluttering the main backpack (and doesn't pop the books out when you die), I understand, but think there's a better way of doing it.
The better way to handle spell/mastery books would be to add 4-6 slots to the CC & EC paperdoll that act as a "book bag". They act as the top level of the backpack, but might not count as items (nor do they count as equipped for purposes of their additional properties). If you click on one of them, it opens. Double-click, and it pops to your hand; non-books in the weapon hand are disarmed, but if both are books they swap locations. Dragging books to and from the bar would work normally. This probably wouldn't be hard to code. The only downside is for those so paranoid they try to carry all their slayer books at once. (seriously, if you know where you're going, you should only need 2 or 3 slayers, max, and keep the rest in your bank or house). I have enough clutter in my pack with Spellweaving, mysticism, magery (plus another in my hand - one for SDI, another the most appropriate slayer) and mastery books. I used to carry a chiv book for emergencies before I realized I didn't really need it anymore, and a necro book for scribing (now only when actually, you know, making necro scrolls).
Rune Books, on the other hand, already have a solution in Rune Atlases. For one of my mains, I combined 10 rune books (some of which were half-full, or had old dead runes to things like guild houses from a decade ago that I didn't need to carry anymore) I had into 3 atlases with room to spare (then sorted all the junk runes into piles either to be reused, or dropped into one of the emptied rune books as "historical locations"). I've already filled a couple of runebook BODs with books that I've emptied that didn't have sentimental value (i.e. weren't exceptional with a name marked).
One Atlas is my personal runes (which includes all the fishmongers), with about 12 slots still open.
A second is an Atlas that goes to every dungeon entrance accessible by spell.
A third is at least one rune to every city (and the entrance tunnels to those that can't be recalled to), with a bunch of extra runes added for special locations. For example, Moonglow has runes for both the bank and the Lyceaum (and to the Papua portal, for Fel Moonglow - Tram Papua has its own rune); Britain's to the banks and to the building where our EM events usually start (which is right outside Castle Blackthorn).
I could drop Britain and Luna runes back into the first book, and not need the other two at all, most days.