If you are new to "Vintage Speed" you might not be aware that this designation did not exist 10 years ago. It's a category that has steadily grown in popularity as the Internet got under way.
The www made searching for information and obsolete hardware very much easier but 10 years ago "Vintage Speed" consisted mainly of Judson Superchargers, Okrasa Kits for 30 and 34hp engines and performance equipment marketed by the original EMPI company in California.
What this means is you are not going to find books written in the last 20 years on the subject, it just too small a market in the larger "VW World". Sure there are books on performance VW engine building but all are aimed at the later twin port Type 1, Type 3 and Type 4 engines.
Virtually everything pertaining to the genre will be found on the Internet Forums and those two books I listed earlier which are collectively considered to be The Bible of Vintage Speed tuning.
Happy hunting...that's half the fun. :d