Below are a few public domain books that look interesting, recently converted to ebook formats and available to download for free from Project Gutenberg. The first one could be a Kindle beach read -- it's been compared to Peyton Place!
Published in 1839 as Darwin's "Journal and Remarks", and also known as Darwin's Journal of Researches, this is his account of the second exploratory voyage of his ship the HMS Beagle, from 1831-1836. A vivid and exciting travel memoir as well as a detailed scientific field journal covering biology, geology, and anthropology, Darwin's notes include comments illustrating his changing views at a time when he was developing his theory of evolution by natural selection
Published in 6 volumes, from 1776-1789, "the work covers the history of the Roman Empire, Europe, and the Catholic Church from 98 to 1590 and discusses the decline of the Roman Empire in the East and West. Because of its relative objectivity and heavy use of primary sources, at the time, its methodology became a model for later historians. This led to Gibbon being called the first "modern historian of ancient Rome."