I frequently get asked to recommend other books for kids to read while waiting for the next Percy Jackson. Below are some ideas. This is by no means a comprehensive list, but these all are books that I have enjoyed, and that have been "vetted" by my sons, who are reluctant readers, or other young readers I've worked with. The grade levels are simply my own rough estimates. They are not set in stone.
The Underland Chronicles, by Suzanne Collins. The first in the series is Gregor the Overlander. A boy falls into a fantastical world under New York. For grades 4th-6th, roughly. My son devoured these.
Jeremy Thatcher, Dragon Hatcher, by Bruce Coville. And many other titles in his Magic Shop series. These are easier reads, ideal for 2nd-4th grade. Both my boys loved these.
The Time Warp Trio series, by John Scieszka. Again, for younger readers, 2nd-4th grade, but very funny and fast-paced. These books got my older son through a difficult 2nd grade year.
The Thief Lord, by Cornelia Funke. My favorite by this author. It's a fast-paced read with a great premise and interesting characters. Grades 4th-7th.
The Alex Rider series, by Anthony Horowitz. A teen thriller series very much like James Bond. Good action and no-nonsense prose. Grades 6th-8th.
Airborn and Skybreaker, by Kenneth Oppel. Adventure novels in the tradition of Treasure Island and Jules Verne, but with a modern sense of storytelling. These take place in a world where airplanes were never invented, and dirigible airships rule the skies. Grades 6th-8th. Again, my son Haley ate these up.
Artemis Fowl, by Eoin Colfer. Lots of fantasy and humor, perfect for middle grade sensibilities. Grades 4th-7th.
Skulduggery Pleasant, by Derek Landy. A funny fantasy/mystery featuring a detective who just happens to be a fire-throwing skeleton. Grades 4th-7th. I don't "blurb" many books, but this one I recommend without hesitation.
The Bartimaeus Trilogy, by Jonathan Stroud. The first title is The Amulet of Samarkand. A more challenging series, but brilliant stuff. A young London magician summons an ancient djinn to help him with his problems. Grades 6th-8th.
Everlost, Downsiders, Dark Fusion series, by Neal Shusterman. My son loves these books -- all great adventures, very accessible. Grades 6th-8th.
Diary of a Wimpy Kid, by Jeff Kinney. I've never heard my son laugh so much while reading a book. This cartoon novel is a real crowd-pleaser, especially for middle grade boys. I loved it too. Just be warned, it's a quick read. Both my son and I polished it off in a few hours and were ready for more. Fortunately, Jeff is making it a series! Grades 4-8.
Warriors, by Erin Hunter. Clans of cats battle for supremacy. Again, my son has really enjoyed these, and he's not necessarily into animal stories. He can burn through a book a day. Fortunately it's a big series! This is one of those series that works well for both boys and girls, which I really appreciate. Grades 4-7.
And for mythology:
Heroes, Gods and Monsters of the Greek Myths, by Bernard Evslin. Much better storytelling than D'Aulaires, in my opinion.
The Usborne Illustrated Guide to Greek Myths and Legends, by Cheryl Evans. A little hard to find, but very accessible, with high-interest illustrations.
And when in doubt, always ask your local librarian or independent bookseller. They are a wealth of information when it comes to kids' books!