Remember that the sharing of files, printers, and other resources is a result of all the devices being connected to the same network, and not necessarily being connected to a router. You could set up a home network with a switch, or a hub. Most modem/router devices have a 4 port switch built in already. Connecting the WAN port on your router to your ADSL line just provides your already established home network with internet connectivity. If your ADSL is down, you home network will still be fine, just none of the machines will be able to access the internet.
Some advice: set up your router to act as a DHCP server. This will automatically assign IP addresses to each of the devices/computers connected to your network. You don't need to manually assign IP addresses. This is tedious.
I assume your modem/router contains 4 ethernet ports as well? If so, then it means your modem is also a switch.