So, what's the importance of Roger Bacon's diary? Canon & conservation of detail both suggest it's something, possibly a horcrux or possibly some other tool of Quirrelmort. This Voldemort is too smart to horcrux his own diary, but this diary would be an awfully convenient trojan horse for him to have (extremely durable, treasured by Harry).

It doesn't seem to produce any sense of Doom, though, which seems to count against the horcrux hypothesis.

Could Quirrell be using it to spy on Harry, to get his curiously accurate priors? Does Harry keep the diary in the pouch which he carries around everywhere, and which he brought to Azkaban, and which the Quirrell cast a spell on and entered in snake mode? If so, it could be a part of Quirrell's current plot. (Or, Quirrell could've done something else with Harry's pouch, which doesn't involve the diary.)