After several months when the number of free books in the Kindle Store (for U.S.-based customers) was basically frozen at about 19,795 titles, there’s big news today. In the past few hours Amazon has again begun adding new free titles to the Kindle Store. These appear to be newly added public domain titles published by Amazon’s own imprint Public Domain Books, and it is not immediately clear whether there is a particular source, library, or catalog for the initial additions that have brought the total catalog to the 20,136 figure seen in this screen shot taken at 10:49 am EST Saturday, February 27, 2010.

Just click here for Amazon’s up-to-date listing of all free Kindle titles at any given time, and use the categories shown in the sidebar at left (also represented in the screenshot to the left here) to drill down for these categories and further subcategories.

Three things worth noting about the process of searching and browsing within this free book list:

• You will need to use the categories shown to be able to search the entire list, since Amazon limits such searches to 4,800 titles, or 400 “screens” of 12 titles each.
• For reasons of its own, Amazon has made it impossible for us to employ the Publication Date sort from the pull-down menu in the upper-right corner to find the most recently listed titles by using a false date such as “March 17, 2006” for thousands of its Public Domain Books titles.
• As implied above, this discussion is focused on Kindle Store listings as they appear to U.S.-based customers.

Ordinarily an increase of 340 or so free public domain titles to a catalog of nearly 20,000 free titles might be of little import, but there are two reasons why these additions pique my curiosity:

• Historically, additions of free public domain titles to the Kindle Store have come in huge increments such as the addition of 7,355 new public domain titles in January 2009 and another 12,500 additional titles later in the year. Each of these dramatic additions occurred within a period of less than a week, so we will be watching in the next few days to see if the first 340 is just a first signal of something much larger to come.
• As we’ve noted previously, the British Library announced recently that it had arranged with Amazon and Microsoft to add 65,000 new free titles to the Kindle Store “this Spring.” There’s no particular reason to believe that the addition of these titles has begun, and a few perfunctory searches show no indication that that’s what going on, but we’re sitting on the edge of our chairs.