I am an omnivorous reader with a strangely retentive memory for trifles.

The speaker characterizes himself as widely and indiscriminately well-read (“omnivorous”), while also admitting that what sticks in his mind is often the small, seemingly unimportant detail (“trifles”). The pairing is revealing: broad reading supplies a vast store of facts, but the real advantage lies in an unusual ability to retain minor particulars—exactly the kind of scraps that can later become decisive clues or connections. The line also carries a note of self-deprecating irony: the memory is “strangely” tuned, not to grand ideas but to minutiae. In Doyle’s world, such minutiae frequently prove more useful than conventional learning.