Physics World has premiered a series of free e-books that cover different topics of interest related to the discipline. They're called Physics World Discovery and so far there are five of them, on topics ranging from dark matter to the use of physics in modern financial market strategy to cancer research.
Did I mention they're free? They march alongside a handful of other series on some scientific topics that range from layperson-friendly to pretty technical.
And did I mention they're free? I'll be busy over here for awhile.