You can also find more information on the topics discussed at the following reference links:
- OSINT resources: http://osintframework.com/
- Maltego user guides and documentation: https://www.paterva.com/web7/docs.php
- Google Cheat Sheet: http://www.googleguide.com/print/adv_op_ref.pdf
- Shodan for penetration testers: https://www.defcon.org/images/defcon-18/dc-18-presentations/Schearer/DEFCON-18-Schearer-SHODAN.pdf