Credo Reference is a good place to start. It includes subject specific references including images, maps, in-depth writings, biographical writings, sound files and more. This is a credible resource and available to you 24/7 from any internet device.
These books are not downloadable to a device. Most readings range from a few paragraphs to a few pages about a certain topic and need to be read on an internet connected device.
Search for "capital punishment"
Use the concept map to find other keywords.
Use the topic search for a general overview.
Use the encyclopedia references to dig a bit deeper.