When it's released, it will definitely be free. Much of the content is already available here on the forum anyway; the idea behind this book would be you don't have to go looking for many different pieces of the puzzle, then work out how they fit together. I'm still considering pdf vs wiki - there's advantages to both, and both can easily be turned into the other. To give an update on this, I stated putting it together, but quickly found it was becoming out of date faster than I could update. I've therefore put it on hold until much closer to release. In the meantime, I'll continue fleshing out the reference guides on this forum.
I was just going through the Modding Reference post and saw this. Absolutely please compile some sort of handbook in pdf. Is there anymore status on this?
Still the plan to create this! However as mentioned things are rapidly changing on the modding front right now, and Uber have announced some documentation being released in January, so I've held off on working on this for now.