How to find US total debt?

The figure provided by that author is widely inaccurate according to the official statistics.

According to the OECD the general government debt to GDP ratio ratio in 2008 was just $102%$. Moreover, here is comparison to other countries:

Moreover, general government is by OECD defined thusly:

General government consists of central, state and local governments and the social security funds controlled by these units. General government accounts presents data on fiscal balance, debt, revenues, expenditure, costs and reserves of governments. Central government consists of the institutional units making up the central government plus those non-profit institutions that are controlled and mainly financed by central government. The political authority of central government extends over the entire economy. Central government can impose taxes on all residents and non-resident units engaged in economic activities within the country.

So it does include central, state and local government and even non-profits that are mainly financed by public funds.

Hence either there are some caveats that you are not mentioning in your question (perhaps it does not include just government debt but also some private debts or present value of unfunded future liabilities) or the author of the book made a mistake.

The World Bank's "World Development Indicators (WDI)" data bank is important and authentic source to get answers for your question.

WDI is the primary World Bank collection of development indicators, compiled from officially recognized international sources.

It includes different series of data (Current & Historical) for countries around the world. Include of course what your are asking about: debt % of GDP.

You can also extract and download the data you selected in different types (Tables, maps, charts) and formats (Excel, CSV)