The Illumined Heart focuses, as the sub-title indicates, on capturing "the vibrant faith of ancient Christians." Granted, this isn't a new theme, but Mathewes-Green does a great job of putting early Christianity into what I would personally consider its proper perspective and explaining how we can apply it today. Her approach in this book is to create a fictional woman, whom she names Anna, and to explain how Anna would have lived as a Christian in the 5th or 6th century, in the Middle East. Of course, there's inevitable projection here, and this certainly isn't intended to be a history book, but the style is effective in helping the modern Christian to gain a better sense of what's missing in modern Christianity (and, goodness knows, that's no short list). Like I said before, it's a fairly short book at just over 100 pages, but it gets the point across. For my part, it stayed in my thoughts for a while.
The Open Door has a different focus altogether. Again, quoting the sub-title, this is about "entering the sanctuary of icons and prayer." If you've ever wondered what all those icons are about, this is the book that provides a clear, albeit brief, explanation. I'll admit that icons have taken some getting used to for me. I have that Protestant sense of "ooohhh, that's bad...idolatry!" so drilled into my head that I needed time to understand the purpose of icons in the Church. Here, Mathewes-Green actually goes into a (fictional) church and describes what the different icons are for in the different parts of the church. She starts with the iconostasis, which is the large screen separating the altar from the main section of the church, and then she describes other common icons that might be seen. (I've included a picture of the iconostasis, with the doors open, below.)
Put in this perspective, icons are not simply art that is intended to evoke a purely emotional response to the beauty of the artwork, but rather they function as ongoing spiritual tools. I don't expect everyone to like them, but after reading more I certainly appreciated their qualities more.
This is another short book (just over 160 pages), but it's also pretty full. I would say that both books serve a fairly distinct audience, so if you're not interested in the topics they're definitely not for you. But I think they're great for those who are looking for a little more information but need an accessible introduction to the topics.