* “Only to the extent that it suits his book” means “only to the extent that the price is favorable enough to justify selling the stock.” In traditional brokerage lingo, the “book” is an investor’s ledger of holdings and trades.
† This may well be the single most important paragraph in Graham’s entire book. In these 113 words Graham sums up his lifetime of experience. You cannot read these words too often; they are like Kryptonite for bear markets. If you keep them close at hand and let them guide you throughout your investing life, you will survive whatever the markets throw at you.