Dialog is a utility to create nice user interfaces to shell scripts, or other scripting languages, such as perl. It is non-graphical (it uses curses) so it can be run in the console or an xterm.ÿ
The C book is no longer in publication. You will not be able to find a copy anywhere (at least I have not been able to find one) But GBDirect has a copy on line here.
Xdialog is designed to be a drop in replacement for the "dialog" or "cdialog" programs. It converts any terminal based program into a program with an X-windows interface. The dialogs are easier to see and use while adding even more functionalities (e.g. with the treeview, the file selector, the edit box, the range box, the help button/box). Because Xdialog uses GTK+, it will also match your desktop theme. Documentation for Xdialog is here.