Hi Lee,
I went to Santorini in May 2015 and it was spectacular, especially if you get a hotel with a view of the caldera

.

- A room with a view (Imerovigli)
I did not get any butterfly reference books, so it was just a case of walking along likely-looking paths.
There is a headland at Imerovigli (Cape Tourlos) which is good for plants and wildlife, but there are many steps.

- Cape Tourlos
I also strongly recommend that you do the cliff-top walk between Fira and Oia which should take around 4 hours. You will find some good spots along the way, including a hilltopping site for several species (including Swallowtail) near Finikia
http://www.travel-to-santorini.com/maps/

- Path near Finikia
Butterflies were only seen in small numbers and were usually difficult to photograph due to the terrain.
Species noted were:
Swallowtail
Lulworth Skipper
Large White
Small White
Bath White
Clouded Yellow
Red Admiral
Meadow Brown
Wall
Lang's Short-tailed Blue
Long-tailed Blue
Small Copper
A truly stunning place..............

- Imerovigli and Firostefani from the Volcano
...............but you need good knees
Vince