UPDATE as on April 8, 2015: The online SQL Server documentation now has a dedicated topic on how you can install and manage Books Online (BOL). See here for the most up-to-date information: https://msdn.microsoft.com/en-us/library/ms166020(v=sql.120).aspx.
If you are wondering why you don’t have a local copy of Books Online for SQL Server 2012, here is the deal. Starting with SQL Server 2012, Microsoft has moved on to a new help technology where only the Help Viewer and related components are installed by the Microsoft SQL Server 2012 setup, which is used for viewing help and managing the help settings; no documentation is installed locally. All of the SQL Server documentation is available online, and you can download the documentation by using the Help Library Manager. But what if the computer you are installing SQL Server on is behind a firewall or proxy and has no access to the Internet.
Well, in that case you can directly download the Books Online for SQL Server 2012 from http://www.microsoft.com/download/en/details.aspx?id=347 to a computer that has Internet connection, and then use the Help Library Manager on the SQL Server computer to install the product doumentation locally. The Books Online download link mentioned earlier has instructions on how to do so.