Hello and welcome to Tom's Hardware Forums.
You already owned everything you needed to achieve your goal with the possible exception of a piece of wire. If you connect to the Internet wirelessly, you don't even need that wire.
Networking the two computers together through the wireless router is simple but you would need to install Quickbooks on your husband's machine for him to be able to access your data relating to that programme.
To save writing unnecessarily detailed instructions, please post back how you connect and if you have a multi licence copy of Quickbooks.