Kevin Beaver wrote Hacking for Dummies and he has just completed the 4th edition. It's a well written comprehensive book on Ethical Hacking. He talks about hacking network hosts, operating systems, applications (communications, databases and websites), password cracking all from the perspective of a security consultant with permission to find the weaknesses in the targets. This can help you understand the dangers your computers or your business systems face from malicious hackers.
NetScanTools Pro is one of the tools he uses to demonstrate how you can assess vulnerabilities. While NetScanTools Pro is not designed as a hacking tool, it does have some tools that you can use to test for vulnerabilities.
Examples used in the book from NetScanTools Pro:
Using Ping Scanner to ping multiple IP addresses to find out what hosts are accessible.
Finding open ports with Port Scanner like the port 53 DNS, NetBIOS port 139, and SQL Server port 1434.
Scanning SNMP or dumping whole system's MIB trees using our SNMP tools.
Using the UDP Packet Flooder as a DoS testing tool.
Windows share enumeration using our Network Shares - SMB tool.
SMTP Relay Testing.
Get the book at dummies.com. ISBN 978-1-118-38093-2