If you use ebooks, you ought to consider the Logos Bible Software systems. This is my prefered ebook version. The base engine is free and the offer a free book each month. Try it out. The text mark-up features are my favorite tools.
Here's another classic I am glad to have available to read and mark up on my iPad. Thanks Logos!
Don’t miss your chance! Get Athanasius’ On the Incarnation of the Word of God for free now!