I've often contemplated on this very thing, given the clientele I deal with. Just how does one trust a "criminal" with the books? Seems a contradiction in terms to me, but then again I'm a law abidin', productive citizen so maybe I don't have the perspective to dwell on such matters. The old sayin' "There's no honor among thieves" comes to mind.
It's a basic problem with the arrangement, either the bookkeeper taking excessive advantage of the situation or flipping to be a Government witness - but there are four primary ways to handle it:
1. The bookkeeper's identity remains a very closely-guarded secret.
2. You as the kingpin have some lever over the bookkeeper so powerful that he would risk rotting in prison for life over whatever alternative thing you can make happen, for instance a clearly-communicated understanding that he won't live long enough to testify if he turns.
3. The kingpin and the bookkeeper are the same guy (Not feasible here, this 'Kid' clearly isn't very good at the crime thing).
4. You have the critical info split up among multiple bookkeepers who do not work together, perhaps also partially encrypted, so that they can't collude to sell you out and none of them has the information necessary to bring you down in one set of hands.