John Frame has written a fine, fine book. As introductions go it is extensive but it also well written, and no where does it assume prior theological knowledge, and when appropriate, Frame explains the theological terms as he goes along.
Frame covers every major doctrinal theme. And although this is not an issue, he does it from a conservative reformed perspective. He is a five point calvinist (limited atonement and double predestination – which is not my position – I am a four point calvinist!!!!) but that does not diminish the clarity, depth and godly work of this book.
If you want to begin serious study of Doctrine this is the place to start – once you have read this, tackling a Grudem, Reymond, Geisler, Hodge or even Berkhoff will be considerably easier and more fruitful.