The easiest way, from what I can see, is to get a wireless access point which you'd point directly to the guest house and connect to your own network. Next step is to get another wireless access point and place it at the guest house. Point it to the other access point and set the points to work in wireless bridge mode. After that you can connect a switch to the guest house AP and build your network around that switch.
This way, on your own network the bridge should be almost invisible and everything should work as one single network. You won't need to modify your main LAN in any way and your main router will still act as your main router. Linksys has a nice article for their products, but the principle is universal.
There are some other options too, but to me at least, they seem more complicated, more expensive on the long run or both.
You could use a single computer with wireless network card instead of guest house access point and have it share Internet connection to wired network. The computer would have to be on all the time and will consume more power than an AP.
Another option would be to use a WiFi router which has a bridge mode or which can be used with DD-WRRT or OpenWRT and have it act as bridge. The plus side is that most consumer WiFi routers are actually combination of router, gateway, access point and a switch. If the router has advanced enough firmware, you can simply not use the router and gateway parts and use the access point and switch. This may be cheaper but will be harder to configure because you'll either have to dig around for a router which has bridge mode in its own firmware or get one which can work with third party firmware and then learn how to defectively use the third party firmware (and that can take some time).
Another option would be to get a single powerful access point at your main home and have guest house computers connect using their own wireless cards. This option looks expensive to me and you will have problems with signal, because it will be more difficult to put numerous computers in good locations.
As a side-note, do get 802.11n access points for your network if you think that computers in both houses will need to use LAN for anything other than just Internet access. On your network, the WiFI bridge is going to be a choke point and you should do our best to minimize its effects.
