Oracle Documentation Set
The first place to start should be the collateral provided by development, there are two new guides introduced for R12 which I think are very good.
This is a great guide as it discusses business, acounting and finance concepts and how they map to the Oracle Financials world. I always reccomend that you start with this so you get an overview of the business problems the Financial suite is trying to solve.
Oracle Financials and Oracle Procurement Functional Upgrade Guide: Release 11i to Release 12 (Part No. B31543-01)
Note - This a new type of upgrade guide which discusses functional issues for business users, not to be confused with the technically oriented other upgrade guide. It discusses how existing set up will be upgraded and should be helpful as you plan the impacts to your business and also consider what new features you want to take advantage of in R12.
There are a couple of good books available that you might want to check out.
Oracle E-Business Suite Financials Handbook, Third Edition
I have the first edition of this book, it is very readable and goes over the concepts and major features. Not sure what will be in the 3rd Edition which is due out mid January, but I assume it will cover R12.
Oracle E-Business Suite 11i; Implementing Core Financial Applications
There is a book available on google books by Susan Foster that is a few years old now but much of the information is still relevant.
Oracle Technology Network Forums
The financials forum on OTN are packed with useful information and there's a very active community there so questions get answered pretty quickly.
You should also check out the blogs on the blogroll, there's a wealth of tips and tricks and detailed information out there. You might also want to subscribe to this blog.
Are there other useful books, papers, website or resources that you find useful? Let me know in the comments section.