Level 5 is as high as the NPO Tadoku books go. The Brookes series goes higher, but the books are still in production. So if you are reading at Level 5 it is time for you to move on to other materials. Essays, nonfiction, and short articles are good choices to build vocabulary and subject expertise. If you are more interested in fiction or manga, the world is open to you but you will also come across a lot of vocabulary you don't already know. That means that you will be stepping out of tadoku's normal rules and should expect to use your dictionary more often. Still, you will want to make sure that you don't look up more than 5 words a page, because reading for pleasure (and education) should be pleasurable and not painful.