Are there any good guides or books on how to start a Bitcoin- and fiat-accepting online business? I'm looking for something that includes going over how to found a company and deal with tax issues and other related human resources issues like payroll.
[...] how to found a company and deal with tax issues and other related human resources issues like payroll.
I don't think such sources are currently available for one simple reason: up to now the bitcoin space is mainly unregulated (including the issue of payroll).
The debate of bitcoin in the legal system is just in the beginning. It's not even that clear (worldwide) whether bitcoin is legally consider money or not. On 2014, Andreas Antonopoulos spoke in front of the Banking, Trade and Commerce committee of the Senate of Canada to address such issues of regulations. I strongly recommend you to watch the full 2hr video.
Guides/books on how to start a Bitcoin-accepting online business?
Supposing that you are already familiar with the basics of Bitcoin, I guess the next step would be deciding how you are gonna accept Bitcoin payments. There are several ways to accept Bitcoin for small businesses. Probably, the most popular ones are Coinbase and BitPay.
If you are still looking for ideas for businesses with bitcoin, an interesting place to start is the Million Killer Apps series of James D'Angelo (Part 1 and Part 2). You might also find useful to subscribe his channel.