Bookkeeping for Note Question

[Peter Halliday]

This is for either those accountants out there, or else experienced investors that have solved this.  From Note investing accounting.  A note I assume would be considered a Loan Receivable.  I know the direct cost of purchase would be included in it.  However, is the value considered what you paid for it, or the unpaid principal balance?  

I assume the bookkeeping for a non-performing loan is different.  But maybe not?