I was reading the last "OSC reviews everything..." column where he mentions this book, The Bridge To Terabithia, and it occured to me that there is a famous russian book, a much older one (1935) with what seems to be a very similar idea. The book is by Leo Kassil, and it is called The Land of Shvambrania. I just wanted to point that out. English translations exists, although I didn't check if it is available in print today.
The book is about two school kids, and how they invent this imaginary country of Shvambrania that is situated on the continent of The Big Tooth.
I haven't heard of The Bridge... until today, so I may be off, but OSC's description surely read very familiar. Check Shvambrania out, great book...
UPDATE: There seems to be the only English edition, long out of print. Here's a link to a library search that I've found.