Hey there,
Congratulations on your book, sounds like a great piece of work. Would love the chance to produce the drawings and graphics you need.
I've got a small team available in the UK (currently awaiting our senior developer to begin work on a new PC game concept). I have a great concept artist (and close friend) who could definitely produce what you want on this. Alternatively I do have someone who practices in charcoal too. Maybe a collaboration could work on this one..!
Would be great to have a chat, I can provide samples prior to agreeing the project, will take half a day (5 hours maximum) to produce.
Please do let me know if you're interested, and good luck with your book irregardless of your choice on this bid!
Best,
Billy