The aim of this book is to help people to get started with Gnucap. Gnucap is an open-source circuit simulator, or circuit analysis package (in short: CAP). This book can also be read on gitbooks: https://mulder-patrick.gitbook.io/gnucap/
Nice overviews on Gnucap can be found in FOSDEM talks:
the gnucap wiki: http://www.gnucap.org/dokuwiki/doku.php?id=gnucap:start
The main Gnucap project page can be found at the GNU servers. Note,this page is a bit older and does tno reflect all the recent development that can be found on in the Gnucap source code.
A useful resource for help is the Gnucap manual: https://www.gnu.org/software/gnucap/gnucap-man.pdf