anyone know more about multi-book? I know you can have a total of 5 books including your primary, but is that in your entire netsuite instance or is it per subsidiary?

Full multi-book means having another full set of transactions with some different configurations. The key word is “full set”. It is not a “full instance” because only some and very particular configurations are different meaning Enable Features configurations and Accounting Preferences are the same for all books. But per subsidiary (and if you select, at the consolidation level) you get a full set of transactions done slightly different based on your requirements such as IFRS rev rec or currency or else.

rememeber, you can’t stack the books on top of each, just one book at a time for financial reporting

If you're thinking you need more than 5 books, there's probably a design problem. Also, please note that if you're doing a "full" book and not just a reporting book, you should hire a consultant or get someone who really knows what they're doing. MB is a little bit tricky and it's easy to create a mess if you do it wrong.