A book value is basically assets - liabilities, a balance sheet position.
For instance, it is known that Saint's borrowed x amount from directors for their stadium build.
If this was say £3m (it would be shown as a credit or negetive figure on the b/s), but the stadium as an asset is worth say £19m (a positive debit figure), then the net position would be £16m.
They'd still have the £3m debt though - which is what the programme was looking at.
London & Huddersfield we know are proped up by an owner and directors loans (debt) with no large assets as such, hence their positions.
Does that make sense?