Books of accounts for 15 months

Yes. This is usually done in the first year after incorporation.

Companies are free to adopt their financial year, but the duration of financial year

"may be less or more than a calendar year, but it shall not exceed fifteen months"

The statements can be prepared for a period not exceeding 15 months as per section 210(4) - definition of the financial year for companies act.

With prior permission from ROC, it can be extended upto 18 months.

Disclose the same in notes to accounts this year.

and next year you have to disclose that 

the financial statements have been prepared for the period from april 01 2010 to March 31 2011. For the previous year ended on 31.03.2010, the financial statements were prepared for the period from DD/MM/YYYY being the date of incorporation of the company to march 31st 2010. hence the figures of current and previous years may not be suitable for comparison.

For Income tax purpose, you cannot make 15 months financials, You have to follow the Fin Min's Financial year.

Whether the above provision applicable in the year of Incorporation or it can be extended to subsequent years ?

It can be extended to other years too.

15- months can also be extended, where financial year of subsidary compnay is not in line with of holding company. For the purpose financial year of subsidary can be extended upto 15 months, by passing a board resolution.

For e.g Financial year of subsidary is ending on December where as for holding compnay it is March, In such case financial year of Subsidary can be extended upto march by passing Board Resolution and without any further approval.

However other provisions of the companies Act has to be complied seaprately, such as conducting AGM...

Regards