Common Lisp interpreter

I'm reading Land of Lisp and the guy uses CLISP (and CLISP specific extensions), so I'm using it for now. But I've heard good things about SBCL and others. Which one would you recommend? Should I stick with CLISP?

Now, where's the interpreter?
sb!eval:eval-in-native-environment is SBCL's interpreter, not the other case, you were looking at when (eq *evaluator-mode* :compile), not the other possibility.

Now look at the sb-eval package contained in full-eval.lisp:

;;;; An interpreting EVAL

;;;; This software is part of the SBCL system. See the README file for
;;;; more information.
;;;;
;;;; This software is derived from the CMU CL system, which was
;;;; written at Carnegie Mellon University and released into the
;;;; public domain. The software is in the public domain and is
;;;; provided with absolutely no warranty. See the COPYING and CREDITS
;;;; files for more information.


And there you will see code handling each special form in full:


(defun %%eval (exp env)
  (cond
    ((symbolp exp)
     ;; CLHS 3.1.2.1.1 Symbols as Forms
     (multiple-value-bind (value kind) (get-variable exp env)
       (ecase kind
         (:variable value)
         (:expansion (%eval value env)))))
    ;; CLHS 3.1.2.1.3 Self-Evaluating Objects
    ((atom exp) exp)
    ;; CLHS 3.1.2.1.2 Conses as Forms
    ((consp exp)
     (case (car exp)
       ;; CLHS 3.1.2.1.2.1 Special Forms
       ((block)                (eval-block (cdr exp) env))
       ((catch)                (eval-catch (cdr exp) env))
       ((eval-when)            (eval-eval-when (cdr exp) env))
       ((flet)                 (eval-flet (cdr exp) env))
       ((function)             (eval-function (cdr exp) env))
       ((go)                   (eval-go (cdr exp) env))
       ((if)                   (eval-if (cdr exp) env))
       ((labels)               (eval-labels (cdr exp) env))
       ((let)                  (eval-let (cdr exp) env))
       ((let*)                 (eval-let* (cdr exp) env))
       ((load-time-value)      (eval-load-time-value (cdr exp) env))
       ((locally)              (eval-locally (cdr exp) env))
       ((macrolet)             (eval-macrolet (cdr exp) env))
       ((multiple-value-call)  (eval-multiple-value-call (cdr exp) env))
       ((multiple-value-prog1) (eval-multiple-value-prog1 (cdr exp) env))
       ((progn)                (eval-progn (cdr exp) env))
       ((progv)                (eval-progv (cdr exp) env))
       ((quote)                (eval-quote (cdr exp) env))
       ((return-from)          (eval-return-from (cdr exp) env))
       ((setq)                 (eval-setq (cdr exp) env))
       ((symbol-macrolet)      (eval-symbol-macrolet (cdr exp) env))
       ((tagbody)              (eval-tagbody (cdr exp) env))
       ((the)                  (eval-the (cdr exp) env))
       ((throw)                (eval-throw (cdr exp) env))
       ((unwind-protect)       (eval-unwind-protect (cdr exp) env))
       ;; SBCL-specific:
       ((sb!ext:truly-the)     (eval-the (cdr exp) env))
       ;; Not a special form, but a macro whose expansion wouldn't be
       ;; handled correctly by the evaluator.
       ((sb!sys:with-pinned-objects) (eval-with-pinned-objects (cdr exp) env))
       (t
        (let ((dispatcher (getf *eval-dispatch-functions* (car exp))))
          (cond
            (dispatcher
             (funcall dispatcher exp env))
            ;; CLHS 3.1.2.1.2.4 Lambda Forms
            ((and (consp (car exp)) (eq (caar exp) 'lambda))
             (interpreted-apply (eval-function (list (car exp)) env)
                                (eval-args (cdr exp) env)))
            (t
             (multiple-value-bind (function kind) (get-function (car exp) env)
               (ecase kind
                 ;; CLHS 3.1.2.1.2.3 Function Forms
                 (:function (%apply function (eval-args (cdr exp) env)))
                 ;; CLHS 3.1.2.1.2.2 Macro Forms
                 (:macro
                  (let ((hook *macroexpand-hook*))
                    ;; Having an interpreted function as the
                    ;; macroexpander hook could cause an infinite
                    ;; loop.
                    (unless (compiled-function-p
                             (etypecase hook
                               (function hook)
                               (symbol (symbol-function hook))))
                      (error 'macroexpand-hook-type-error
                             :datum hook
                             :expected-type 'compiled-function))
                    (%eval (funcall hook
                                    function
                                    exp
                                    (env-native-lexenv env))
                           env)))))))))))))

(defun %eval (exp env)
  (incf *eval-calls*)
  (if *eval-verbose*
      ;; Dynamically binding *EVAL-LEVEL* will prevent tail call
      ;; optimization. So only do it when its value will be used for
      ;; printing debug output.
      (let ((*eval-level* (1+ *eval-level*)))
        (let ((*print-circle* t))
          (format t "~&~vA~S~%" *eval-level* "" `(%eval ,exp)))
        (%%eval exp env))
      (%%eval exp env)))

(defun %apply (fun args)
  (etypecase fun
    (interpreted-function (interpreted-apply fun args))
    (function (apply fun args))
    (symbol (apply fun args))))

(defun interpreted-apply (fun args)
  (let ((lambda-list (interpreted-function-lambda-list fun))
        (env (interpreted-function-env fun))
        (body (interpreted-function-body fun))
        (declarations (interpreted-function-declarations fun)))
    (call-with-new-env-full-parsing
     env lambda-list args declarations
     #'(lambda (env)
         (eval-progn body env)))))