Is there a word for a 'Dictionary about flowers'?
Yes: it's called a florilegium.
a. literal. A collection or selection of flowers; transferred in the title of a book
b. A collection of the flowers of literature, an anthology.
Etymology: modern Latin, < flōrilegus flower-culling, < flōr(i)-, flōs flower + legĕre to gather; a literal rendering of Greek ἀνθολόγιον anthology n., after the analogy of spīcilegium.
OED (may be paywalled)
OED has it in frequency band 3: "This word belongs in Frequency Band 3. Band 3 contains words which occur between 0.01 and 0.1 times per million words in typical modern English usage. These words are not commonly found in general text types like novels and newspapers, but at the same they are not overly opaque or obscure."
While the -legium part may not be immediately obvious, flori- is likely to indicate something to do with flowers.
Botanists call a reference book of the plants of a particular region a flora However, in everyday speech most people would just call it a wild flower book.
The common term here is nature guide. This is how the publishers of such works decribe them, often with some additional words to identify the region or other specialization.
For example, U.S. National Wildlife Federation uses this word to describe their collection of apps.
https://www.nwf.org/natureguides
Most readers will have encountered these books (and now apps) at some point in their lives. They already know that nature guides are specialized, so the action of looking up a flower implies that the nature guide in question is a plant guide or flower guide.
A google search for “nature guide” will return multiple examples.
The term florilegium was proposed by another poster. This is a valid word, but it’s too rarely used to create much of an image in most readers’ minds. When the reader looks it up, the definition will look something like this entry from the Wikipedia:
“ In medieval Latin a florilegium (plural florilegia) was a compilation of excerpts from other writings.The word is from the Latin flos (flower) and legere (to gather): literally a gathering of flowers, or collection of fine extracts from the body of a larger work...
Florilegium is also the name of a journal, several stores, and an early music ensemble. Its use as a synonym for nature guide is not common.
A writer’s choice of word should be based on what they intend to convey to the reader. Nature guide can be pictured immediately by most readers. Florilegium, however, can be built up by the writer into almost anything consistent with its wide range of meanings.