I came across this wonderful PDF from ALA’s Digital Content Working Group (DCWG) showing the availability of Big Six (soon to be Big Five) ebook titles to libraries. Random House, Harper Collins, Hachette, Macmillan, Penguin, and Simon & Schuster are included. It highlights the amount of content available for purchase, license/use terms, and the vendor platforms where content is available. Robert C. Maier is maintaining the document and his last update was May 15, 2013, so the information is pretty up-to-date. Robert based his chart on one started at Library Journal earlier this year. He welcomes comments via email at robert_maier[at]comcast.net
Thanks, Robert, for maintaining this information.