VW-TP (Transport Protocol) is the correct term. CAN-Bus, like Ethernet or RS232, is only responsible for the hardware-based transmission. The Golf IV has TP1.6, while the Golf V/Touran has TP2.0. The Passat 3B/BG has TP1.6, and from the Passat 3C onwards, it uses TP2.0. This gives a rough indication of when the changeover occurred, although exceptions are common.
Information is not freely available. Everything is "non-disclosure." Those interested in this topic simply log the traffic and try to decipher it themselves. One resource, besides Google, would be the
CanHack Forum. You can also check out
OSEK/VDX, specifically the specifications for NM and COM. This is very similar to the VW-TP and will give you an idea of what to expect. Another source of information is Bosch's MC-Net, which is very similar to VW-TP1.6. About two years ago, it could still be ordered for free from Vector Informatik. However, I believe they have removed the corresponding link from their website.
It's also important to note that transport protocols are unlikely to be very helpful to you, as they primarily deal with the transport of data. In essence, they involve breaking down data into 7-byte blocks, reassembling them later, performing error checking, and retrying if necessary.
The interesting part is the usage data, which is not part of the TP specifications, but which can vary from model to model - although they are generally quite similar. Here, "sniffing"

is the only way to get that information.
Greetings,
Axel.