It would be a noun, and I've been trying to look for if there is anything even similar, but I have sadly come up empty with Dictionary.com, Thesaurus.com, and the Reverse-Dictionary, with a general Google search.
A 'dictionary about flowers' would simply be a book about flowers, their meanings, genus, name, and maybe they would have a picture or two (As most books of this type do)
It would be used in a sentence like
He'd looked up the flower in Rachel's [insert word] just then, and found it to be an Aconitum.
I'd much prefer if it was single-word; I've considered phrases like 'Botanical dictionary/encyclopedia/compendium' or 'Flower catalog' and the likes, but they don't fit the single-word criteria and either have no ring, or the meaning of which are too broad or slightly off.