I am just starting out in this area: here is what I have. I would welcome comments on them (evaluations, notes of strengths or failings, suggestions for better ones).

General works

Todd, Malcolm The Early Germans (1992, Blackwell, Oxford). ISBN 0-631-19904-7

Roman Empire and Germans

Carroll, Maureen Romans, Celts and Germans: the German Provinces of Rome (2001, Tempus, Stroud) ISBN 0-7524-1912-9

King, Anthony Roman Gaul and Germany (1990, University of California Press, Berkley) ISBN 0-520-06989-7

Wolfram, Herwig (translated into English by Thomas Dunlap) The Roman Empire and its Germanic Peoples (1997, University of California Press, Berkley) ISBN 0-520-08511-6

Costume and equipment

Wilcox, Peter; Embleton, G.A. Rome's Enemies 1: Germanics and Dacians (1982, Osprey Men at Arms series #129, Oxford) ISBN0-85045-473-5

Religion

Davidson, Hilda Ellis Roles of the Northern Goddess (1998, Routledge, London) ISBN 0-415-13611-3