If you’re looking for a guide that covers a wide variety of different species (and most of them not US-specific, although this is a American book), you’ve found it.
This is not a comprehensive guide to drawing flowers, each demonstration being limited to a single page, or a spread at most, but all of the information is there, with generous illustrations and notes on how specific details were done and on the colours used.
There’s a lot of useful information here and it’s concisely presented, so that those who want to work by example rather than wading through acres of text will be well served.