R
I'm sure you must have looked into this already, but have you considered an online self-publication with somewhere like Blurb? I self-published a book of photographs with them, and the quality was superb, though I chose the 'luxury everything' settings, so it wasn't cheap, but for mostly text, it'll be a manageable price.
There's no stock to pre-order, no up-front costs, and the niche market/readers you're aiming it at are best positioned to buy it direct online via portals like this. You can gauge an interest here, so it could sell quite a few copies. It gets past the bookshop problem, and the need to fork out a fortune to pay the printer to do the first run.
A set of ISBN numbers can be bought for £107, I think it was, and that would let you do hardback and paperback editions on the same set of regs.
Cheers
Phil.