I've been looking at the same. I can help with some links, but am still well in advance of my 20 hour, so don't look to follow in my footsteps.
Here's a link to a solenoid and switch combo from Blue Sea Systems (well thought of in the marine industry):
http://bluesea.com/products/7650
Poke around at the above link and you'll find highly detailed wiring diagrams and explinations of how the 'Add a Battery' package works.
Here's the cheapest place I could find to buy it:
http://pkys.com/Blue_Sea/Switches.htm
In looking at the '07 stock FW wiring harness for a Volvo GXi, I find two sets of cables attached to the positive terminal of the original battery. One set appears to be a 1/0 AWG wire to the starter, and the other set consists of a bundle of 14 AWGG or smaller wires I assume to be the house wiring. It's tempting to seperate House and starting batteries by these two bundles. I haven't spent any serious time tracing these cable sets, I imagine that the alternator must tie into the wire to the starter.
Anybody know if it's as simple as running the bundle set to a house battery and the 1/0 AWG to the starter battery, to achieve a starter battery (charging) and house battery (isolated) system? I'm just looking for a starting point, and then would wire in the Add a Battery system.