(let ((a 1) (b (+ a 1))) (message a))
This throws the error
Debugger entered--Lisp error: (void-variable a)
What\'s
The canonical way is to use let* (also note that I added a %s format string to your message form):
%s
message
(let* ((a 1) (b (+ a 1))) (message "%s" a))
The let* function allows you to reference other variables that have previously been defined.
let*