Lisp isn’t too difficult to learn. The syntax is off-putting to some people, but what helped me adapt to Lisp syntax quickly was my familiarity with RPN calculators; I used an HP graphing calculator in college and I regularly use dc in Unix. In my opinion, the best way to start is to learn Scheme and work through The Structure and Interpretation of Computer Programs, which is a very nice read even for experienced programmers. Scheme is a nice small language, and many undergraduates have written Scheme interpreters in Scheme. After mastering Scheme I recommend moving on to Common Lisp, which is a large language with many features that are not common in more popular languages, such as conditions for error handling, the Common Lisp Object System and its support for multiple dispatch, the metaobject protocol, support for image-based development, and support for REPL-based development and debugging.