DD6 has been enjoying the Magic School Bus series of Science Chapter Books. She just finished #17, Food Chain Frenzy. First she read it by herself, then we re-read it aloud together. As we read, we made a list of any words she didn't know or were hard to pronounce (chlorophyll, photosynthesis, illuminating, phytoplankton, carbohydrates, etc.) Later I typed these up, showed how each word divides into syllables so she can practice saying it, and gave a brief definition for each.
This was time consuming for me, but she's really excited to start a collection of "big words" from various books she reads. And she is learning a lot from it--expanding her vocabulary, as well as learning new concepts. DS8 wants to read the book and look over the words, too.
On the lighter side, DD6 is also enjoying a series called the Cobble Street Cousins by Cynthia Rylant. These are really quick, easy, "fluff" reading for her (age guide is 7-9), not challenging, but they're sweet books for a little girl. These would be great for a 4-5 year old girl who is starting to read chapter books.