Net Block ( Book Value ) = Gross Block (cost) - __________.
Gross Block Means - Total Value of Asset ( Its Original Purchase price+any cap ex)
Net Block Means - Original Value Of Asset( As Above) - Till Date Charged Depreciation
Normally "Accumulated depreciation account " is maintained which is credited by the depreciation charged each year on the asset of the company , Once you debit current period depreciation , such depreciation shall be transferred to Accumulated Depreciation Account.