For any Leanpub book, on the "Sales" page under "Store" you can also see Paid, Free, and Total purchases.
To find the "Sales" page for your book, go here:
https://leanpub.com/YOUR_BOOK/sales_30_days
...making sure to replace YOUR_BOOK with your book's unique web address.
You can also get information on all purchases of any book on the "Individual Purchases" page under the "Store" menu, e.g.:
https://leanpub.com/YOUR_BOOK/individual_purchases
There, you can download a CSV file for all your purchases, and you could sort all the free purchases in Excel.
You can find links to both of these pages on your book's Overview page:
