This gorgeous cover for the paperback of Nicola Upson's An Expert in Murder caught my eye. Shame I've already read it. This is the first in the proposed series starring Josephine Tey and was reviewed on Euro Crime, here and here. The paperback is out in February. Though AEIM is set in the 1930s, the following series are set (or at least begin) in the 1920s:
Carola Dunn's Black Ship is out any day and is the 17th of the Daisy Dalrymple books.
Australian Kerry Greenwood has finally got a UK publisher (Orion) for her Honourable Phyrne Fisher series which saves me from importing them any more. Murder in the Dark is the 16th in the series and was published earlier this month (in the UK).
The Winter Ground is the fourth in the Dandy Gilver (society sleuth) series by Catriona McPherson and is due out next month. (Cover shown is from January's paperback edition.)