All of the official Acrobat JavaScript Documentation: http://www.adobe.com/devnet/acrobat/

Adobe XML Specs
http://partners.adobe.com/asn/tech/pdf/xmlformspec.jsp
http://partners.adobe.com/public/developer/xml/index_arch.html

XFA Scripting Info
http://www.adobe.com/support/techdocs/330467.html

XFA Object Spec (Basically the XFA JavaScript DOM reference)
  ** look for the link in the right hand side bar
http://partners.adobe.com/public/developer/xml/topic.html

FDF Toolkit (Free Stand-Alone SDK for working with FDF files)
http://partners.adobe.com/public/developer/acrobat/fdf/topic.html

Acrobat 3D Documents
http://partners.adobe.com/public/developer/pdf/topic_3d.html

PDF Specification and the like:
http://partners.adobe.com/public/developer/acrobat/sdk/index_doc.html#api

PDF File, Command Line Open Parameters: very very useful for linking to a PDF
http://partners.adobe.com/public/developer/acrobat/index_advanced.html#aaud

Best JavaScript Book is  "The Definitive Guide" from O'Reilly:
http://www.oreilly.com/catalog/jscript4

 

pdfscripting.com: http://www.pdfscripting.com
This is our favorite site for information on scripting in and around the Acrobat/PDF platform. Of course it's our site and we support it ;) You'll find video tutorials, articles, sample code, sample PDFs, automation tools, and Acrobat plug-ins, all covering a wide range of activities from interactive form features to process/workflow automation. And we are constantly expanding it with new information. This is a paid for membership site, but it's also chuck full of free content. Check it out!!

4xPDF: http://www.4xpdf.com/
A Blog and information portal by long time PDF entrepreneur Karl De Abrew, "News, Articles, Tips and Rants on the topic we know best: PDF". This is fresh new stuff from someone who knows. An amazing amount of information, news, and links about the PDF world, slanted towards the non-Adobe side. Updated regularly, this is the place to go to get the latest news!!

4xPDF Forum: http://help.4xpdf.com/
This is a WiKi style forum, billed as the Ultimage PDF Knowledge Exchange. This site is chuck full of useful tid-bits of information on everthing PDF. And of course you can ask you own questions, post infomation, or add comments to the existing threads.

Acrobat User Groups: http://www.acrobatusers.com
Acrobat Central for Users.  Nice looking and easy to use site. Articles, Tutorials, Blogs, Samples, and more for everything from entry level to expert information. 

AbracadabraPDF:  http://www.abracadabrapdf.net
An excellent set of PDF examples and some free tools. However it's a French site so you're on you're own in figuring out what's what.  Look for the "galleries" link to find the PDFs

Acumen Training and Journal:  http://www.acumentraining.com
Very Excellent information on Acrobat, PDF, and JavaScript Programming as well as lots of sample documents and working examples.  And of course PostScript.

Acrotips, http://www.acrotips.com
By the creator of the Acrotip, Dave Wraight, this site offers a large collection of PDF related links, articles, tips, and the Office2PDF products. Includes a search engine for tips.

Citation Software, http://www.CitationSoftware.com
Variable Data Printing and Publishing products and services.  Hands on help to get your product/process/workflow setup and running, but they also sell and provide information on a host of products useful for this area of PDF. The site also has lots of tips and facts on PDF; provides industry news coverage; and information, libraries & toolkits for PDF Developers.

Document Solutions, http://www.document-solutions.com/
Duff Johnson, founder and CEO of Document Solutions is well known in the PDF community and is recognized as an expert in the area of PDF archival, content deployment, and accessibility.  Many excellent articles on a variety of PDF topics can be viewed at this site.

PDF Zone, http://www.pdfzone.com
Comprehensive resource for all things PDF. Includes a Buyer's Guide with search engine, discussion forums, news, tips and techniques, free newsletter, and more.

PDF Worker, http://www.pdf-worker.com
Extensive database of PDF software tools including editorial evaluations, ratings, and user commentary.

Planet PDF, http://www.planetpdf.com
Comprehensive resource for PDF including forums, product search engine, on-line store, news, tips, learning center, free newsletter, and more.