While there are quite a few books out there on how to hot rod your VW, and on Vintage VWs, there are very few books on how to hot rod your vintage VW! (Other than books that recommend tossing out the vintage engine/chassis and starting with something newer) (which isn't such a bad idea)
The two vintage VW books that I was told about are "Making the Volkswagen Go! Power and Speed Complete" by Henry Elfrink, and "Souping the Volkswagen" by Dick Morgan. Click here for pictures of these two books, with a couple examles of what's included in Morgan's book. (I doubt you'll find them at any garage sales, but at least now you'll know what to look for!)
Both of these books provide a lot of information on vintage speed parts along with the basics of engine mods. Elfrink's book is the more complete of the two, covering the entire vehicle, while Morgan's book covers only the engine. Both books were printed around 1960, and both are now, not surprisingly, out of print.
I looked for over a year before finding a copy of either one, then within a month I found copies of both. They are out there.
"Without Guesswork" is a source of data for VW engines. There are different versions for the various years. Beware, the 1960 version covers engines built from August 1960, so it will NOT contain data for a 36hp engine.
Conversion Books - for those who want to put Porsche Power (356/912 or 914) in their VW.
"Secrets of the Inner Circle" By Harry Pellow. This is a well known manual on how to rebuild a 356 engine. Also includes a bunch more information, including sections on putting a VW engine into a 356, Putting a 356 engine into a VW, and which parts will interchange between a 356 and a VW.
"The Maestro's Little Spec Book" Another Harry Pellow book. More information on the 356 engines. (Some repeated information with "Secrets")
"The ABC's (and 912s) of Porsche Engines" And another Pellow book with more information on the 356/912 family of engines. This one basically covers the development of the various components over the years. Again, some information is repeated in the other books. Note that this one is only available on disk in text format. Kind of annoying actually, but with a little creative editing it can be useful. Note that it was scanned from a printed copy and the OCR software has made a few mistakes. If you are easily bothered by things like that, don't bother with the disks, try to find a printed copy somewhere.
"Type IV Conversion Manual". By Joe Cali. Great source of information of what's required to convert the type IV engine over to upright cooling. (note that you can buy conversion kits now. They are somewhat expensive I suppose, but they are certainly an interesting alternative to building your own)