Here is the documentation that exists for xbook. Please check the source code for specifics. (Go figure)
To install XBook just unzip the distribution zip file somewhere under your webserver's document root directory. Unzipping will create a directory called guestbook (which you can rename to anything you want). For example if you unzip the file directly into the document root directory, you can then begin using XBook by pointing your browser to http://<yoursite>/guestbook/
Although XBook runs right out of the box, within the source code there are several optional configuration steps that allow you to customize various things, for example every time somebody posts a new entry alert emails can be sent to an email address that you define. Also included is the ability to customize the "header" and "footer", i.e. the HTML that comes before and after the guestbook content.
An area where I could improve this script is in making it easier to integrate into an existing website. Right now (12/4/2002) all you need to do is open index.php, locate the header and footer functions, and customize the HTML code found there. Clear as mud eh? Okay, note to self, build a UI for customizing the header and footer.
One other area where improvements could be made is in customizing the colors. Right now (12/4/2002) all you need to do is open the style sheet in files/css/guestbook.php and edit the hex codes found there. What? Isn't everybody a CSS whiz? Well, maybe I'll work on making a UI for this too.
The admin console allows you to edit or delete messages and change the access password. For admin console access go to adminlogin.php under the guestbook directory. The default password is xbook. Please make sure to change this to something else.