National Solver is a magazine aimed at the amateur sleuth community, apparently a large demographic in the Tackleford universe. It was founded in 1899, and the current price is £4. A recent issue featured its list of the "15 under 15", headed by Katya Schloss. There must be a lot of bad guys who would have gotten away with it if it weren't for those meddling kids.
Charlotte Grote reads this magazine (it is unknown whether she subscribes or buys it on the newsstand).