Like you said, it was last week's Free Fridays selection. It was free until Tuesday. Free Fridays aren't guaranteed to stick around beyond that Friday.
Promotional free e-books come and go very rapidly. Some are free for only a few days, or less. This week we had two that were free for 2 days, and one that was free for just a few hours. 16 more were scheduled to be free for just one day (Jane Austen's birthday) but have been extended to 2 days. There's a thread on this forum where we try to keep track of the comings and goings: http://bookclubs.barnesandnoble.com/t5/NOOKbook-Discussion/Free-NOOKbook-summary-thread-please-no-OT/m-p/761176#M16813
If you want a free e-book, you should move quickly. They don't stick around.
This week's selection (Ninja by Jon F. Merz) shows it listed as $1.99 after I click the "Download for Free" button. It's been showing that way since early this morning. ??