is squid 2.x configuration still roughly the same as for squid 3.x? I have the o'reilly squid book which covers squid 2.x - will this book still be useful for me to configure squid 3.x, or has too much changed between the versions? Is there any documentation describing the configuration differences between 2.x and 3.x?

Here you can look at the Squid configuration directives on this page maintained by the Squid project. This page shows the different directives and which versions of squid they're available in. The page is titled: Squid configuration directives.

!ss of squid config directives page