Starting in third grade, most readers can move beyond repetitive series and handle books with more complex plots. This is a great age for Harry Potter, Narnia, and other complex books. SO MANY great books are out there for kids this age it is hard to know where to start.
One caution: many kids at this age will have a reading level that is beyond their emotional capacity. Just because they CAN read it does not mean they SHOULD. Some kids are more tender than others, and while one may have no trouble with creepy or dark stuff, like the Spiderwicks or A Series of Unfortunate Events, others will lose sleep. Same goes for romance, which can be bewildering and upsetting for kids before they are developmentally ready.
Know your own kid, and take the time to read along with them.
This is a great age to start graphic novels, which are especially appealing for reluctant readers. Talk about a great work-reward ratio! Pictures support text, and graphic books are lively and visually appealing. Not only that, but many of these books are quite complex and offer a real literary journey for kids. Try one! It might surprise you!
Graphic novels are tricky in terms of being age-appropriate, because they all look like candy to an 8-year-old, and some have quite sophisticated plots, even graphic violence and sexual content. Most have age recommendations on the back. Pay attention!
Some starter titles for this age group include Bone by Jeff Smith, Amulet by Kazu Kibuishi, The Secret Science Alliance by Eleanor Davis, and of course, the classics, Tintin by Herge and Asterix by Albert Uderzo.