I'm opening a custom computer company, but don't know much about troubleshooting the larger hardware issues that go outside of a cable not being plugged in properly. Does anyone have any good book or website suggestions that I could read up on?

I'd look for resources on A+ certification which focuses on basic computer hardware repair. There's stuff online, as well as books you can buy. Also, the website TechRepublic has a lot of articles and references on a variety of computer topics.

I'll be a contractor for you.

It's not easy learning enough about computer hardware to be able to troubleshoot effectively. I speak from experience - I used to train folks wanting to switch to an entry level IT support job. It was a compressed, 7 month program. To learn on your own, get yourself any new A+ genre textbook. I'd stay away from study guides only because they gloss over the deeper topics you'll likely want to learn. 'Upgrading & Repairing PCs' by Mueller is my textbook of choice. Get the absolute newest edition. But like others have said, you're in for a steep learning curve. And you'll be confronted with new generations of technology every 18-36 months.

Find a CompTIA A+ certification study guide. While you're at it, you could even get your certification!