Credit through bank transfer

If its in nature of income, then he must offer it to tax.

Where the assessee is not able to explain the nature & source of such receipt, then it shall attract tax liability being "unexplained money" u/s 69 or 69A r.w.s 115BBE. (section 68 is concerned with unexplained credits in  one's "books of accounts")

Regarding the 2nd question:

Bank Pass book (a.k.a Bank Statement) is not books of account for the purpose of Section 68.

In CIT, Poona v. Bhaichand H. Gandhi 141 ITR 67 (Bom.) it was held that the pass book supplied by the bank to the assessee cannot be regarded as the book of the assessee, that is, a book maintained by the assessee or under his instructions. Therefore a cash credit for the previous year shown in the assessee’s bank pass book but not shown in the cash book maintained by the assessee for that year, does not fall within the ambit of Section 68 of Income Tax Act, 1961.

Thus if AO finds any unexplained transaction in the bank passbook of the assessee then same can be taxed as unexplained money under Section 69A of the Act.

Rate of Taxability: –

As per Section 115BBE of the Income Tax Act, if the total income of an assessee includes any income referred to in Section 68, Section 69, Section 69A, Section 69B, Section 69C and Section 69D, the income tax would be payable at the rate of 30% (plus surcharge, education cess and secondary and higher secondary education cess as applicable). Notwithstanding anything contained in this Act, no deduction in respect of any expenditure or allowance shall be allowed to the assessee under any provision of the Act, in computing his income referred to in Section 68, Section 69, Section 69A, Section 69B, Section 69C and Section 69D.