B's "I don't know" is an admission of not being fully able to meet the quality maxim, which is the basis for 'perhaps' in the implicature. The excuse also alerts A not to infer anything from B's not simply giving clear directions to the desired section, which is to say that B seems to offer too small a quantity of information. Relevance is also in play: B talking about psycholinguistics books when A has asked about sociolinguistics books can be taken as co-operative by reasoning that B must think that it could be relevant that books in another long- named branch of linguistics are there; specialist branches of the same subject could have similar names.