Think the only real options left are.
1) wait and hope the book shows up.
2) take CGC'S offer and accept the loss.
3) file police report for stolen property. Take out small claims court case against UPS for the loss, and leave BBB report. Share CGC CASE # of the graded book and hope someone spots it and try to recover it.
None of these options are great, but unfortunately this is the predicament you currently find yourself. Others have mentioned that this was a choice of which carrier to use and the services they do(or dont provide/insure)
You took a chance not having it shipped through a carrier that covers insurance / loss. Which is further exacerbated by not carrying private insurance to cover potential loss.
These options are laid out on CGC'S website.
Yes it sucks and I am sure a lot of us cringe at the thought of loss, but it is a risk we all take when sending off our collectibles.
It is always best to do everything you can to mitigate the loss should the unthinkable occur.
The only other thing you can do is chalk whatever loss you have incurred in this as paying for education on how to proceed in the future.