After re-reading my review and my comment regarding Dumbledore, I began to wonder if the fact that Harry 'found' the book might have something to do with Dumbldore's manipulations. In which case the instruction that his friends not contact him might have more relevance. Rather than openly admit to Harry that he was a mage, Dumbledore allows him to discover it himself. Dumbledore then arranges for the least useful of his watchers to be on duty at the relevant times to allow Harry to organise his 'escape'; it would be his way.
Suitably Machiavellian methinks. Vale Clivius