Since 2012 release, Civil 3D API Developer's Guide and API reference document is available online. Here are the relevant links -
AutoCAD Civil 3D 2013 API Developer's Guide & API Reference online
AutoCAD Civil 3D 2012 API Developer's Guide & API Reference online
If you are looking for a down-loadable copy of the Civil 3D API Reference document, here you go -
Download AutoCAD Civil 3D 2013 API Reference Document
Download AutoCAD Civil 3D 2012 API Reference Document
Please note that, down-loadable versions are one-time only post and they won't be updated on regular basis. Hence we suggest to refer the Online API Reference document for up-to date version.
