Middle School Fantasy series. Goblins. Boundary (tree/ bush) between humans and goblins. Bratty girl lives in very nice house. Illustrations. Two books: blue cover, red cover. Read 2009-2012.

[Capn]

There's a Goblins list, but I don't think I saw it - worth a scroll, if you'd like: https://www.goodreads.com/list/show/4...

[Kris]

Jordan, I copied some book details to the header/ topic title. Feel free to edit it.

[Capn]

Jordan wrote: "Hi! I'm trying to find this series and it is driving me crazy.

I read this book 10-15 years ago, probably in middle school. I can confidently say there are two books, one is blue and one is red. T..."

Did you mean 'Changelings' (or challenges)? If the former, there's also this list:
Changelings https://www.goodreads.com/list/show/3...

Series on that list include:
Changeling, which is a series of two books and is set in Central Park, Manhattan (basketball court, maybe?): https://www.goodreads.com/series/9531...
&
Switched from the "Trylle" series: https://www.goodreads.com/series/4892...
&
The Iron King from the series of the same name: https://www.goodreads.com/series/4732...
&
many others - have a look!

Blodwen and the Guardians shares some features, but it's a standalone, afaik.

[Geneva]

Fablehaven by Brandon Mull is about a brother and sister who visit their Grandparents big home in the middle of a magical reserve. Past the fence line is a forest with magical creatures. The first book is green, but the second and third are blue and then red, and in the third book there are some satyrs playing on an old tennis court.

[Capn]

Geneva's suggestion: Fablehaven

[Kris]

Geneva's suggestion of Fablehaven by Brandon Mull(book 1) looks very promising.

Google Books preview:

https://books.google.ca/books?id=w7Ff... There are different cover images.

Quote from the book:

The wall of bushes extended as far as Kendra could see in either direction. "How do we get around it?"
"Through it. You'll see." (...)
Before her lay a pristine pond, a couple of hundred yards across, with a small, verdant island at the center. A series of elaborate gazebos surrounded the pond...

"Fablehaven" series - https://www.goodreads.com/series/4336...