From No Limit Hold 'em: Theory and Practice by David Sklansky and Ed Miller.
Concept No. 35: Unusually small bets tend to be made either with a big hand (a suck-in bet) or with a bluff (a cheap stab at the pot). With one pair, your opponents will usually either check or bet a larger amount.
This is a strange concept because it contains no advice. I think the information is correct, but, to me, it seems only slightly useful.
In my experience, I think it's true that when someone makes a small bet it means it's slightly more likely that he has a big hand, and slightly less likely that he has one pair, but it's not a very significant difference. Depending on the game you are playing in, this concept could be off the mark for a stereotypical player; different types of plays are more or less popular in different venues, and this can also change over time. Also, once you have figured out what a particular opponent likes to do, the generalization made by this concept will be obsolete for that opponent; this concept applies only to players you do not know much about, and for them, bet-size is only a weak indication of hand-type.
Supposing that your opponent's bet-size really were a strong indicator of the type of hand he held, this information would be quite useful, but not devastatingly so. On the one hand, if you knew your opponent either had a big hand or a bluff, you could decide whether to fold right away or just call; raising would never be a good play in this situation. If, on the other hand, you knew your opponent held a pair, it could be a good play for you to raise either for value or as a bluff, but it would still be unclear how your opponent would react to this raise or if he would continue to bet his pair on future streets.
With this concept, Sklansky and Miller are pointing out a very marginal interpretation of how to read an opponent's action, and it's dependent (as they admit in their discussion) on who your opponent is. They offer no insight into how to use this information, although I can accept that they probably consider it outside the scope of this concept (or maybe they just think the answer is obvious).