All Scribes know that currently certain glyphs can only be learned through these Books though Blizzard announced that in the future all glyphs can be learned through daily research.
Also, all scribes know that the Book of Glyph Mastery is a world drop from any mob on Northrend. However, most players now operate in the newer content (i.e. Cataclysm) and hence there are not many of these on the Auction House. So, currently at least these are in very low supply.
However, on occasion new content arrives where these books are in demand and the prices spike as Scribes rush to take advantage of the demand for the new glyph. In such situations it pays to have a stake of four tucked away. If only two books are required you can take advantage of the demand for the new glyph by having them learned and ready for sale immediately, and then flip the other two as the demand for the glyphs peaks which will coincide with the demand for the Books.
For sure, Blizzard may render these Books worthless or at least allow all glyphs to be learned through daily cool downs. For the former the books will be worth nil for the latter there will still be demand from new scribes to accelerate their learning. Either way, a stack of 4 books is a worthwhile form of insurance.
On my server there are normally two to four for sale on the Auction House at any one time and they currently go for about 300 gold but have been known to occasionally drop to 100 gold. When this happens i buy that book and then assuming my current stock is at least 5 books i will repost when the price spikes to 400 gold.
My primary aim here is to maintain a stack of four as insurance against new content placing more glyphs in the game to learn. Flipping is a bonus, not the aim.
for this and other ideas on the glyph market please follow the link to the free Croda's Inscription Gold Guide