Feynman&#x27;s &quot;Quantum Mechanics and Path Integrals&quot; is about $18 on Amazon<p><a href="http:&#x2F;&#x2F;www.amazon.com&#x2F;Quantum-Mechanics-Path-Integrals-Emended&#x2F;dp&#x2F;0486477223" rel="nofollow">http:&#x2F;&#x2F;www.amazon.com&#x2F;Quantum-Mechanics-Path-Integrals-Emend...</a><p>That is a serious graduate level book - written in his own style.