My 4 and 5 year olds use the Collins Complete British Wildlife book the most, they just find the section they want and look until they think they've got the right picture.
The other day I also got them the Natural History of the British isles by Parragon, that was £8 and they really like it, although it's not one to carry about
Look for well illustrated and set out books and show the child how to use the book, they don't have to be able to read it all, just identify from the pictures, the reading will develop as time goes on and you ask what it says and as they become more independent...
Good luck :biggthump