I'll check Daniel Shanks' book out. But the OP was interested in reaching a level where the proof of such summation is not `black magic` - at least that was my impression - which implies reaching a level considerably higher than the minimum... Hardy would be helpful, and is at a sufficiently higher level than future Project Euler questions he might encounter e.g. those involving continued fractions, or Diophantine equations, or even modular arithmetic etc.