I frequently go to thrift stores looking for early reader books or board books that are in good condition. These are super affordable (around 50 cents each) and sturdy. Many times they are textured and I may add only a letter to a page or one word using sticky back clear paper. If they aren't textured, I will add something very simple like a feather to represent a bird. I use hot glue and/or velcro to affix items to the pages. Velcro is nice for items that a child can remove and explore further.
These are perfect for early learners. They are just starting to explore books, textures, turning pages, searching for braille. Here are some examples.