I also like Audubon’s best for general guides, but my absolute favorite are area-specific guides–ones for my state or area. It makes it much easier and faster for children to use (adults too!) Especially since I live in the upper Midwest, and guides are full of plants and animals that live only in Florida, or California, or the sea coast—-LOL!
For birds, however, I try to get as many guides as possible, and I prefer ones that have good lists of field markings rather than pictures. A terrific picture of a bird sitting on a branch doesn’t do me much good if I’m trying to ID the bird I just saw fly through the trees with white tips on the spread ends of its tail! LOL