NOTE: Remember just because a books age rating is something like 8+ or 9+ doesn't mean the book or the story is childish, it just means that the language and graphics are appropriate for those ages as well as older ages, I highly recommend all of these books.
ANOTHER NOTE: If you are thinking about reading Killing Sound by Paul Southern, please take the age rating seriously and be careful because it does contain graphic descriptions of gore and some heaver topics that may not be suitable if you aren't in the in a good mental state.