We have briefly discussed the technologies used in customizing UAG, and you should strive to understand them to as high a level as possible. The following is a list of recommended books to provide you with a deeper understanding of the topics of ASP, HTML, CSS, JavaScript, Windows Server, Information Security, ISAPI, and COM. None of these are absolutely mandatory, but strongly recommended.
Sams Teach Yourself Active Server Pages 3.0 in 21 Days by Scott Mitchell and James Atkinson, ISBN 978-0672318634
Head First HTML with CSS & XHTML by Eric T Freeman and Elisabeth Freeman, ISBN 978-0596101978
JavaScript: The Definitive Guide by David Flanagan, ISBN 978-0596101992
XML in a Nutshell by Elliotte Rusty Harold and W. Scott Means, ISBN 978-0596007645
Windows Server 2008 R2 Unleashed by Rand Morimoto, Michael Noel, Omar Droubi, Ross Mistry, ISBN 978-0672330926
Hacking Exposed, Web Applications by Joel Scambray, ISBN 978-0071740647
Writing Secure Code by Michael Howard and David LeBlanc, ISBN 978-0735617223
Essential WinInet: Developing Applications Using the Windows Internet API with RAS, ISAPI, ASP, and COM by Aaron Skonnard, ISBN 978-0201379365