Yale University is currently working on a new dictionary project.
Contact MArtin Benjamin (Hickory@Yalevm.yale.edu) who is moderating
the list, "Kamusi-L".
There was a new edition of the Kamusi printed last year by Oxford (this
is the original UDSM-inst. of Kisw. research publication)--1994 (I think).
There is also a dictionary (Kiswahili-English) published by the MSO
Training Centre for Development Co-Operation in Arusha. It's a useful
source with citations that are extremely idiomatic. I believe it was
developed by Tanzanians for use in training employees of NGOs (and
missionaries?). I have a copy and use it frequently.