Project Gutenberg may have long since been surpassed by the Internet Archive and Google Books, but the granddaddy of all digitization programs is still chugging along and uploading more free ebooks.
Today they announced that they'd released their 40 thousandth title. (Of course, this does include some duplicates and withdrawn titles so the count is slightly off.)
Book #40k was originally published in 1905 with the title Extinct birds : an attempt to unite in one volume a short account of those birds which have become extinct in historical times : that is, within the last six or seven hundred years: to which are added a few which still exist, but are on the verge of extinction. The author was the second Baron Rothschild, and according to Wikipedia the contents of the ebook were drawn from Rothschild's lecture On extinct and vanishing birds, which was published in the Proceedings of the 4th International Ornithological Congress 1905 in London.
The original print run for this book was limited to only 300 copies, and it had 45 color paintings and 4 black and white sketches. You can find it on Project Gutenberg, and the Internet Archive has their own copy.
This book is also a very good example of how times have changed. It went from only being available in research libraries and private collections to being available any time and every where.
BTW, I'd recommend that you go get the copy from the IA. The image and general ebook quality of the PG produced ebook is disappointing, and that goes double when you consider that the IA has had this ebook since 2008.