[kazinator]

Skimming through it and seeing stuff like this doesn't leave much headroom for a high opinion, at least from a hard-line Lisp and C expert point of view:

  /* excerpt from builtin_op function */

  while (a->count > 0) {

    /* Pop the next element */
    lval* y = lval_pop(a, 0);

    if (strcmp(op, "+") == 0) { x->num += y->num; }
    if (strcmp(op, "-") == 0) { x->num -= y->num; }
    if (strcmp(op, "*") == 0) { x->num *= y->num; }
    if (strcmp(op, "/") == 0) {
      if (y->num == 0) {
        lval_del(x); lval_del(y);
        x = lval_err("Division By Zero!"); break;
      }
      x->num /= y->num;
    }

    lval_del(y);
  }

For people who are newbies to programming and newbies to C, this sort of book can provide stimulating activities and motivation. Ultimately they will probably be unsatisfied with their results (but then nobody is ever satisfied with their results no matter what, if they are newbies in programming and C, struggling to make something).