Complex sentence: Those on flowers were beautifully illustrated, some if not all by Mabel Step (his wife?): in particular a "pocket guide" entitled "Wayside and Wayside Blossoms", published in 1905.
Split into simpler sentences: Those on flowers were beautifully illustrated, some if not all by Mabel Step who may have been his wife. These include a "pocket guide" entitled "Wayside and Wayside Blossoms" published in 1905.