The list now shows the *approximate* number of days each book has been free in the past months.
For example, "Days free in past months: 1~2~3~3" means the book has been:
free for 1 day in the current month (excluding today)
free for 2 days the previous month
free for 3 days two months ago, and
free for 3 days three months ago.
Unfortunately, I don't believe the cheap kindle books (those I list daily for under $3) becomes free much. But if it were free in the past few months, you would now know (and therefore might want to wait a little while to see if it will become free again).
To monitor if a particular book is free, I would suggest using camelcamelcamel's kindle product price watch. It works the exam for books as it is for any other Amazon products.