My book is included in the top 10 results if you Google "pretzel books for children", and so are these!
Pretzel by Margret + H.A. Rey (perhaps most famous for Curious George) about the adventures of the longest dachshund in the world, who is able to twist himself into a pretzel and saves the day for a friend.
Walter the Baker by Eric Carle - another origin story of the pretzel where a baker must use water instead of milk in his famous rolls one day. The Duke is disgusted and threatens to banish the baker unless he can take the same dough and make a good-tasting roll that "the rising sun can shine through three times," which becomes the pretzel. This charming story was told to Eric as a boy by his grandmother.
The Magic Pretzel, a chapter book by Daniel Pinkwater is for grades 2-4 and is about a group of 4th graders who search for the magic pretzel of Alexander the Great, the only thing known to truly cure werewolves!
There are other pretzel books, but My Daddy is a Pretzel rounds out this list for today. It's a cute book about yoga poses that you can see here.