Depends on how deep you wanna get into mentalism.
Impression device = A few mentalism effects.
13 Steps = Strong foundation in the art of mentalism in general and all it's techniques and secrets, and the best starting place if you plan to specialize in mentalism.
Basically, 13 Steps is for people who are really serious about mentalism.
That last line should read more along the lines of "13 steps is for people who are really serious about getting into
mentalism". It is a beginners book. But still holds valuable references for a seasoned mentalist... on occasion.