I teach 8th grade science, and I wouldn't pass up a chance to plug _Surely you're joking, Mr Feynman_ to the kids. It may be a little higher level for some, but many kids will grasp and love stories about the physicist who also played bongos, played with a Samba band in Brazil, worked on the A-bomb, picked locks, cracked safes, drew nudes, nearly got into fights in bars, rode in a car with a spy, and the list goes on.
_The Beak of the Finch_ was great for high schoolers, but the advanced eighth grader with interest in the subject might enjoy it. Books about glacier hikers and mountain climbers (_Into Thin Air_, I think) almost (or actually) losing their lives are a big draw for kids, and heavy on the earth science. I don't know that any Volcanologists have written any quality work, but if they have, my students would read it.
Don't hesitate to stretch their reading level a bit, and look for material OUTSIDE of the middle school library - Some kids rightly view the available selection as 'Lame'. You'll get points for keeping a selection of cool books in your in-class library.
--Matt