"The Case of the Terrified Typist" by Erle Stanley Gardner

Yeah.  Definitely a new comfort-read series.
The plot of this one really kept me guessing.  It had several twists that I greatly enjoyed :-) 
Perry Mason has a lot of work to get done in a short amount of time, so he sends word to a temp agency to send an extra typist because Della Street and Gertie are unable to keep up.  A typist arrives, proves to be brilliant at her job, but disappears.  And then there are all sorts of robberies and treasure discoveries and accusations of murders, culminating ...