I highly recommend Peter Norvig's example Lisp interpreters. He has Python and Java implementations and they are both very accessible, so you can digest them in

I highly recommend Peter Norvig's example Lisp interpreters. He has Python and Java implementations and they are both very accessible, so you can digest them in a few hours' time. "Production" language interpreters tend to have so much complexity that it can be hard even for an expert to find the essence of the interpreter.