Cross-updating makes use of UUCP and related programs. For users who want to know more about these program and fine tune the communications, we can recommend the following books:

Using UUCP and Usenet - A Nutshell Handbook; Publisher: O'Reilly & Associates; ISBN 0-937175-10-2
Managing UUCP and Usenet - A Nutshell Handbook; Publisher: O'Reilly & Associates; ISBN 0-937175-48-X

For those interested in shell/transfer scripts :

Unix Shell Programming (2nd Edition); Kochan & Wood; Publisher: Hayden books; ISBN 0-672-48448-X