Non-fiction for an 8yo boy

Hi!  I am looking for independent reading books for my 8yo son.  I posted awhile back that we just finished Dan Frontier and Billy and Blaze.  He really liked both of those series, but we have flopped on any other series.  We are back to him grumbling about reading again!   I loved the book ideas that everyone gave me, but most of them were too long or had too many words on a page.  He gets really hung up about the size of the print and how many words are on a page.  His reading level is about high 3rd grade.  But, he will ditch a book very quickly and say it is too hard if he thinks there are too many words on the page.  Of course, I can make him read them, but I don’t want to force him to read if I don’t have to do it!  He seems to be really interested in non-fiction books lately.  I have found a few readers that he enjoyed that were pretty good.  He likes science (mostly animals, extreme weather, and “cool” stuff)  and biographies (explorers, inventors, and “cool” people). “Cool” is his new favorite word!   But, those were rare finds and it took us a long time in the library to hunt them down!  Does anyone know of a non-fiction series of readers that would still fall in the living book category?