While you won't be able to assemble a set of full gear using this as a reference, this is an unbelievably good primer on the basics of German field equipment. Covers all basic items of kit very well, and gives dates of introduction, common colour shades, and extent of issue (ie to whom the items were issued and for what purposes).
Illustrations and colour plates top notch - clear and well laid out, rich with detail. Photos very good also, many unpublished ones, and of a uniformly high quality. Equipment and artifact photos a little poorly done, obviously taken outside in bright sunlight, making for harsh shadows and poor detail.
Overall, possibly the best basic guide out there, certainly for the money. Next step up would be Lee's SOLDAT series, which is more of an investment and lacks the clarity of Volstad's colour plates, but goes into more detail from a collector's point of view regarding variants and how to tell fakes from real items.