Using Float.pi that early in the manual is problematic because modules are only introduced one chapter later. Replacing π with e might be a good idea to avoid giving the impression that π is not defined in the standard library.

let pi = 2.0 *. asin 1.0;;
let pi = 2.0 *. acos 0.0;;
let pi = 4.0 *. atan 1.0;;
let pi = 2.0 *. Complex.(arg i);;
let pi = 2.0 *. Complex.((log i).im);;

Adding e might be an option; but I think that it is far less straigthforward than π.
Contrarily to π, e is really easy to access with exp 1..
Moreover, e is rarely used as an isolated constant by itelf. The closest example than I can think of right now is Stirling formula n! ~ √(2πn) (n/e) ^ (-n) but this can also be written as √(2πn) exp ( n log n - n). To the point that I have personally used the Euler-Mascheroni γ more often than e (as an isolated constant).