Anyone who is serious about antenna design should get the ARRL book. Search the site, and you'll see this actually gets recommended often.
As to the infamous Pringle's antenna, you are correct. It is the wrong diameter. Search the O'Reilly web site, and you will find that it was based on another antenna, that was properly designed. The designer of the Pringle's (Rob Flickenger) can did not have the correct size can (a nut can, if I recall) so he tried a Pringles, and it worked well enough for him to publish the results.
So why it does work at all? It isn't a wave guide, contrary to what some have said. The "foil" coating apparently is not heavy enough to produce a guide function. As you have said, it is actually closer to an enclosed yagi than anything else. Depending on a number of factors, including contruction skill and pure luck, people have reported anywhere from 2 to 12dB (dBi
.)
You must remember however, that many people who are reporting such gains are using software graphs such as the one included as part of NetStumbler to measure the results. Most do not have access to use, and may not be aware of the existence, of instuments to do such things. So the actual results, as measured on a spectum analyzer, might be different than the reported values.