The McCreight book is all right, not much theory, mainly "do this, drill this, sand this"
I heartily recommend the Ron Lake book but be prepared to pay up to $85-100 on Amazon. The Lake book has a section from Lake, Clay and Centofontane. They all make a lock back differently and it's interesting to see how they take such a different path to get to the same end. The Lake chapter I found more useful as far as teaching how to design a lockback.
I originally bought the Lake book way back when for $15, lost it during a move and had to pay $85 for it. It's worth it ifyou want to make lock backs though.
Edited to add: One thing I found helpful was to buy some lockbacks and folders from flea markets and yard sales then disassemble them for patterns.