Who drew the elaborate title pages for some of the volumes?

The later volumes have elaborate title pages  or frontispieces which seem to have a purely decorative effect. This  web page shows all the relevant pages. As it can be seen, these are all in typical late-Victorian art style.

Thompson and Renfrew in 1999 discuss these pages:

'Volumes 3-9 have striking coloured title plates, no two alike, but each basically an elaborate art nouveau plant design, perhaps derived from a book-plate, with the General's arms in a shield and sometimes with a helm and scroll with his motto 'Aequam servare metnem' inserted ('keep a level head'). There is also a title 'Objects collected by General Pitt-Rivers from 189- to 189-'. These ingenious title plates had been initiated by Waldo Johnson for volumes 3 and 4, but the remaining five are signed (very discreetly) by W.H. Evans. Evans is not recorded among the General's staff and the plates were perhaps commissioned. They constitute a very bold opening to the volumes.' [Thompson and Renfrew, 1999: 385-8

The identity of Evans is not confirmed but he might be the artist called W.H. Evans, A.R.C.A. (Lond.), who is mentioned here, as being the Principal of the Brighton School of Art in the 1920s and who illuminated a book. Another source gives his name as William H. Evans.

 AP, March 2010, updated December 2010