OK, it has value- so does anything else a VoPov PC can carry, little though that value may be. In weapons alone, he could legally carry a Heavy Mace (12gp), a Hvy Xbow and some bolts (52gp).
Yes, I know that's orders of magnitude less than a 3500gp spellbook.
But this isn't ostentatious material consumption- this is something essential to what the class is.
After all, would you rule that a VoPov Bard could ONLY be a poet or singer, or would you allow him to carry an instrument?
Besides, the BoED doesn't say that a VoPov PC can't own or carry expensive components:
BoED p30
A character who has forsaken material posesions may find himself at a marked disadvantatge when it comes to certain necessary expenses, such as expensive material components. One option is for ascetic characters to beg components from party members...
I read that to mean that since he hasn't got much money and little of value for trade, he will not be able to
purchase anything at the alchemist & wizards' supply shop. He'll have to find other, non mercantile means to acquire spell components, including finding them himself while adventuring. Perhaps he can get his components by doing odd jobs for the apothecary- sweeping, cooking, repairing the roof, etc., in exchange for 3 hots, a cot, and some components.
I
also take that to mean that, by demeanor, a VoPov PC might find it embarrassing to be holding onto 250gp of diamond dust while the family of 4 that is putting him up for the night can barely make ends meet. Instead, that PC would/should surrender that dust in gratitude for the aid given to him out of the family's necessities, or may not even LEARN that spell, since each casting requires conspicuous consumption.
Since the VoPov description expressly limits possessions to simple weapons, simple clothes, 1 day's food, and a spell component pouch, RAW, you could argue that a spellbook is simply not permitted, and thus, barring a houserule or late adoption of the Vow (meaning the PC could accumulate feats like Spell Mastery and Innate Spell), a Wizard would be effectively barred from taking VoPov.
I would counter, however, that it does not seem to be written with such intent. Otherwise, Clerics would be similarly (but not so entirely) discomfited by this feat because nowhere in the Feat's text does it allow an ascetic to carry a holy symbol- definitely something of value that would be essential to the class' functions (like Turning Undead). Bards would be unable to carry instruments.