A good Guidebook
Miscellaneous: Lonely Planet are always fantastic, and as Lefkas isnt such a big tourist island I found it difficult to find a good guidebook.
This one mainly covering Corfu, but also the rest of the Ionians was the best guidebook that I could find.
A few things to bring
Luggage and bags: A small backpack would be handy for when you go hiking up to the waterfall and in the mountains.
Clothing/Shoes/Weather Gear: The beach is really rocky as you walk in, so a cheap pair of flip flops or sandals would be ideal.
We were here in July and even though we were told it was unusual it did rain for a few days,not cold just wet so its a good idea to bring light raingear.
Toiletries and Medical Supplies: You can buy all you need in the pharmacy or any of the supermarkets
Photo Equipment: Bring loads of film, it is so beautiful here you will take tons of photos!
Miscellaneous: Books, its so relaxing bring a few books to read.