Hi all, can anybody help me to find literature that takes the reader on a step-by-step path from non-relativistic quantum theory to relativistic quantum theory? I imagine something like that: it starts with a single harmonic oscillator, analyzes the non-harmonic oscillator (does the state-space change somehow?), then goes on with coupled harmonic oscillators or free quantum fields, arriving at coupled non-harmonic oscillators, both non-relativistic and relativistic. A similar path could be followed from non-relativistic quantum mechanics via non-relativistic multi-particle physics to relativistic multi-particle physics. Just to give a rough idea. The reason is: I am just wondering where in the line from one-particle QM to relativistic QFT the difficulties enter. Is it the multi-particle aspect? Or the interaction aspect? Or the relativity aspect? All combinations of these aspects could be carefully analyzed. Any suggestions of literature are appreciated!