VPN protocols

[koro78 () hotmail co uk]

Hello everybody,
I am currently doing a research on VPN protocols.
It would be nice if someone could give me any ideas about which protocol (PPTP, L2TP, IPSEC, SSL, SSH)fits better for 
different business requirements. I mean that i am interested in knowing for what purposes you would choose IPSEC, SSL, 
PPTP etc depending on different requirements.
I am not interested in specific VPN products but the only i need to know is how to choose the right protocol.
Also it would be nice if you could suggest any book or article that includes real VPN case studies.
Thank you in advance for your help,
Alex