Sadly the use of VHDL and Verilog has removed the need to understand much more than the differences between Moore and Mealey state machines, you don't really even have to understand anything about DeMorgan's theorem or delve into the philosophy of logic because the complier sorts it all out for you, which is a real shame I think because its an interesting subject.
When I were a lad we had to reduce more than 4 terms of logic using reams of cross referenced tables to find the prime implicants and had to be able to convert AND/OR logic to NAND/NOR logic without having to think about it and we had to be able to explain why it works from first principles which I doubt most logic designers would be able to do now.I bet most woulndn't know a Quine-McCluskey reduction if it bit them.

This page about the philosophy of logic has some interesting links but it is a HUGE subject and only a small bit of it is about first order logic http://en.wikipedia.org/wiki/Philosophy_of_logic