Having just discovered my Kindle app (mine's on an Android tablet, they are available for most/all tables and smart phones) I took a look at what's available at Amazon's Kindle Store - and noticed *many* books with a $0.00 price tag - including:
Insectivorous Plants by Charles Darwin (Kindle Edition - May 1, 2004) -
On the Origin of Species By Means of Natural Selection, or, the Preservation of Favoured Races in the Struggle for Life by Charles Darwin (Kindle Edition - Mar 1, 1998)
The Descent of Man by Charles Darwin (Kindle Edition - Mar 24, 2011) -
Lots more! They may be interesting and/or useful.