Borderboys,
I have a vast 'keepering library but I have to admit most of my titles are from way back when gin traps were legal and the Euston System was still in its infancy!
The modern books don't tend to have much detail or rely on the principle of releasing more game to cope with losses through bad husbandry (vermin, disease, straying etc)
One book I would recommend would be Game Conservation by Charles Coles which is a big volume but is well worth its weight as a standard reference to the subject, with its roots set in Game Conservancy heritage.
They do show up on eBay, i