Of course, one of the most valuable resources to the XenApp administrator is the XenApp community itself with their fora, wikis, and videos. Citrix plays a big part in this itself.
You can visit the official website of Citrix at http://www.citrix.com.
You can also refer to Citrix eDocs at http://support.citrix.com/proddocs/topic/xenapp/ps-library-wrapper.html. This web portal is very useful– I know, we should always "read the manual", but in this case it is true.
CitrixTV contains videos from Citrix, both marketing and technical, relating to their product range and can be found at http://www.citrix.com/tv/.
Personally, I maintain a list of Citrix tutorials, as well as other technologies and these videos and blogs can be found on my WordPress site at http://wp.theurbanpenguin.com.
Brian Madden is a VDI expert who blogs relentlessly on Citrix and other technologies at http://www.brianmadden.com, and he has to be on your must visit sites.
You can refer to the blog of Andrew Morgan at http://andrewmorgan.ie. Andrew is an independent developer of ThinKiosk, a thin client computing solution, who knows huge amounts about Citrix and related remote access technologies.
Ingmar Verheij is one of the most influential people in the XenApp community. His blog entries are well respected and shared at http://www.ingmarverheij.com/.
Trond Eirik Haavarstein maintains the XenApp blog. A Citrix professional for many years, his site is current and up-to-date at http://www.xenappblog.com.
Follow Andrew Mallett on http://twitter.com/#!/theurbapenguin.
Follow Citrix XenApp Team at http://twitter.com/#!/xenappjunkie.
For more Citrix and IT goodies information, follow Packt Publishing at http://twitter.com/#!/packtpub.