My older 2 kids love books -- 2 year old daughter and 5 year old son. Both of them love the Cat in the Hat Learning Series..... There's a Map in My Lap, There's No Place Like Space, Wish for a Fish, Oh Say Can You Seed, etc. There are over a dozen books in the series. They are educational but it's the fun-to-read rhyming style Dr Suess is known for.
Curious George is also a hit with my son and my 2 year old likes Richard Scarry. They also both like the Berenstein Bears books. Do you know their favorite movies or characters? There are countless paperback storybooks based on movies and tv shows -- Thomas the Train, Strawberry Shortcake, Sesame Street, Spiderman, etc.
My 5 year old is reading now and he likes the simple Level 1 or Level 2 readers. Bookstores like Barnes and Noble have a section just for those readers in the kid department. He likes the super-hero themed ones he can read but they also have Princess stories and other storylines like ones about dogs or cats or going to school that your neice might like.