Some missing favourites (to be found at
http://www.gutenberg.org/ and/or
http://www.manybooks.net/):
- Charlie Chan (& others) by Earl Derr Biggers
- Big 'Un and Little 'Un (& others) by Louis Tracy
(also some ripping adventures)
- Hamilton Cleek, "the man of the forty faces" by Thomas W. Hanshew
(thoroughly unbelievable, but with some fine plots)
- The Man in The Corner by Baroness Emma Orczy
(a favourite of Tommy and Tuppence)
- Mr. Reader by Edgar Wallace: his Mind is worth looking into!
Now, if one can stand SF/Fantasy (some people cannot) there are some excellent mystery/detective plots by Lois McMaster Bujold, featuring:
- Miles Vorkosigan, in Mountains of Mourning, Cetaganda, Memory, and all the last Lord Auditor Vorkosigan books;
- Ista, in Paladin of Souls.