When we are studying any given topic, I like to have several types of great books on hand. I usually look for a couple of good chapter books to read aloud, some really special picture books to read aloud, some information-rich books like Eyewitness books or encyclopedias, and one or two books at each child's own reading level. Finding good, interesting, well-written books on science or history topics that are suitable for beginning readers can be tricky, but we have been very happy with the All Aboard Reading series of books. They are great for younger readers, but are never babyish - in fact, I use them for read-alouds also. They often lend themselves very well to notebooking ideas as well!
One of my children's favorite titles is Knights. If you are studying the Middle Ages, you have got to get this book! This book tells the story of a boy named William (Our William just loves this!) as he leaves home to become a page and works his way up to knighthood. It is a wonderful book with marvelous illustrations, many of which just beg to be notebooked. It includes castle cutaways, labeled armor and weapons, and more. Terrific!