Bible Truth Publishers has lots of free digital reading material available, including Macintosh’s Notes on the Pentateuch.
More eBooks can be found at Stem Publishing. I cannot guarantee that all of this material is doctrinally sound, but there is some good material there.
DiskAid (for i-Devices)
Download files to your iPhone via USB. (Useful for eBooks that are in text form)
FileApp (for i-Devices)
Allows you to read PDF, MS Office and text files, among others.