The OAPEN Library contains freely accessible academic books, mainly in the area of Humanities and Social Sciences, and published in the last few years. Downloads are PDF.
"The Biodiversity Heritage Library (BHL) is a consortium of natural history and botanical libraries that cooperate to digitize and make accessible the legacy literature of biodiversity held in their collections and to make that literature available for open access and responsible use as a part of a global biodiversity commons.”
Coverage is from 1450 to current with downloads available in PDF format among others. A link is included to Internet Archive for more e-reader formats.