Middle School: Dogs Best Friend by James Patterson

Brand new but there are a couple of very tiny dents to cover.

It's a dog-eat-dog world, and Rafe Khatchadorian is just trying to live in it. Life in middle school is finally starting to seem bearable-until Rafe spots his grandmother standing on the free-meal line at the local soup kitchen. In order to help bring in more money, Rafe concocts a brilliant plan: a dog-walking business that soon turns into a huge money-making neighborhood empire. He'll even have extra cash to buy his own WormHole Deluxe Multi-Platform GameBox!
 
That is, until two terrible twins launch the Great Dog War by sabotaging Rafe's flyers and stealing his customers. Plus, his bratty kid sister Georgia has skipped a grade, so now she's in all of Rafe's classes. Throw a kid a bone!