Unfortunately, there is not much else I have seen has vocabulary and grammatical explanations as you go along (ie, a reader format).
If I may offer a "shameless plug," you might look at my Koine Greek Reader as a stepping stone to Whitacre's reader. KGR has selections from the Apostolic Fathers and early creeds--with vocab, parsing, and grammar help. You can find it on amazon for less than $20: http://www.amazon.com/Koine-Greek-Reade ... 825424429/
Kregel, 2007, ISBN: 9780825424427