For UK based wildlife photographers, I have posted you a few key reference books that I like and have in my collection across a range of our subjects. These are vital in subject identification for those species that we do not recognise.
Until Kindle get round to a colour screen, then reference books are best as precisely that - real books.
The Birders bible
For other subjects,
A few more stocking fillers for you (or your partner to buy for you)