Note: due to the fact that this list is huge, I will not be doing summaries. Just comments.
The Mysterious Benedict Society by Trenton Lee Stewart
Gemma Doyle series by Libba Bray
Septimus Heap series by Angie Sage
Pretty Little Liars by Sara Shepard
Warriors by Erin Hunter
Series of Unfortunate Events by Lemony Snicket
Cover Comments: I love the style of these. And the drawings are fun too.
The Mother-Daughter Book Club by Heather Vogel Frederick
Cover Comments: Stacks of things! Most of them books! And different bright colors! And the characters! Me gusta.
The Spiderwick Chronicles by Holly Black
Cover Comments: This is one of the few series’ covers where the characters actually match what I see in my mind. Beautifully done.
Inkworld Trilogy by Cornelia Funke
Cover Comments: These are exquisite covers. I LOVE them.
Summaries provided by Goodreads. Except not on this one. Links are to Goodreads.