IMO showering as a function is poorly explored and I've never read any
book which discusses the issues and comes up with recommendations.
Most simply deal with aesthetics, impermeability of surfaces, or cost
and difficulty of installation.

To give you an idea of what I'm talking about just take the handles of
the average shower faucet: short lever, long lever, cross, ball, ...
Should it turn right or left? What level of sensitivity should the
faucet have? This is not opinion! There is a right answer although the
right answer may not be implementable because of costs or aesthetics.

Does anyone know of a book or website that discusses bathroom design
considerations from a perspective of functionality and ergonomics?