State tax resources

As businesses become increasingly virtual and states become stricter about nexus. I find myself doing more multistate entity returns. It seems like most of the tax resources don't go into as much detail with entities as they do with individual taxation.

What is your favorite resource for the ins and outs of state business and entity taxation?

I'm looking for things like:

- Entity filing requirements
- Regularly updated nexus requirements for income, sales, payroll tax
- Corp filing and property tax requirements would be a bonus
- Any quirks of credits for other state taxes
- Passthrough taxation details
- Entity tax planning tips
- Links to instructions & forms

The Tax Book does provide some of this information at entity level. At least, it provides enough guidance to help direct you on further research, as needed. An example is their State Business Tax Summary.

There is a resource that has TREMENDOUS information--it is either from TR or Wolters Kluwer, called State Tax Library, but it costs a lot of money. You can obtain access, though, if you form an accountant partner relationship with ADP...