When you sell an asset at a price higher than the original cost of asset how is it treated in the books?

If it's a fixed asset, it's treated as a gain. The amount of gain is booked in the "Gain on Sale of Assets" revenue account (or something similar). The typical entry would be:

Debit Cash (or Accounts Receivable)

Credit Fixed Assets (or whatever account held the asset you're selling)

Debit Accumulated Depreciation (if your asset had any A/D)

Credit Gain on Sale

If you're talking about selling inventory, that's a different matter.