Adam is so excellent, he's like my favorite character. Behind Crowley. And Aziraphale. It's a really simply cosplay really, you'd just have to... Make your hair short, blonde and curly, either via wig or magical hair styling, and then wear a t-shirt, old jeans, and trainers. All the while looking like a ten(?)-year-old boy (I forget how old Adam is sldkfjsd). So short is A-OK MAN.
I'll lend you the book. It's in awful condition so you don't need to worry about being careful with it, either!