Which is the better introduction to Lisp: On Lisp (by pg), SICP, or another text (maybe on Common Lisp)?
Or is it better to just go straight to Clojure these days?
On Lisp is an advanced text about Lisp macros, not recommended as an introduction (but highly recommended if you've written Common Lisp).
SICP is great but not really about Lisp.
I recommend Norvig's PAIP [1] or Graham's ANSI Common Lisp [2].
Another book that's often recommended is Practical Common Lisp but I think it's dated and hasn't aged well. It's also nowhere near as mind expanding as PAIP/Graham's books.
[1] https://github.com/norvig/paip-lisp
[2] PDFs can be found on Google
Clojure is not recommended as it is very different to Lisp.
Lisp code needs to be entirely rewritten before it will run on Clojure. That is not true for Common Lisp, Emacs Lisp and even Scheme.