What can a blank journal represent? We mean, it's blank—there's nothing to it.
Freak is giving Max a fresh start, telling him, "I want you to fill it up with our adventures" (23.36). This is Max's chance to start a new history and put everything that happened with his mom and dad behind him.
But of course it's not that simple. The whole thing becomes a huge challenge for Max because he's convinced he can't write. So Freak gives him a shmoop in the right direction: "Just write it all down like you're talking. Put in all the fun we had, the cool things we did" (23.41). Max has grown a lot since the two boys first met, and this is his chance to truly put the pre-Freak the Mighty Max behind him.
When Freak dies, Max throws the book aside. He reverts to his old habits and hides in the down under. But once he's finished his major mourning, the book is waiting for him. That's right: his fresh start is still there. And this time, Max takes the step forward.
Do you think Freak gave Max the book for this exact purpose?