Kodak's "Photoguide"
a small book, roughly four inches by five inches
not cheap for its size; my 2001 edition was 15$
but...
literally packed with useful information even for digital photographers.
things like Kelvin equivalent light sources and exposure calculators and DOF calculators
(I hope they have updated it for digital formats!)
its only 44 pages, but every page has something you may eventually want to know.
Even if only to follow a conversation.
...mischance nothing, thus idle woe.
all beer has food value...
all food does not have beer value.