Recently, I did a deep dive on InfoPath 2007 and Forms Services for a couple of customers.  They asked for a number of references that would get them up to speed, so here is what I shared with them.  For the most part they are 2007-related but I also included a couple of the "classics" from 2003.  If you have one you really like that's not on the list, send it to me.

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InfoPath General Overview

InfoPath Forms in Office SharePoint Server 2007

InfoPath 2007, Forms Server, MOSS 2007

InfoPath 2007: Designing Form Templates With The New Features Of InfoPath

Hosting the InfoPath 2007 Form Editing Environment in a Custom Web Form

Hosting InfoPath 2007 Forms in WSS v3 / MOSS 2007 Pages

Creating Browser-Enabled Forms with InfoPath 2007

How to Deploy an InfoPath 2007 Form Containing Managed Code to a Browser-Enabled WSS v3 Library

Rendering an InfoPath 2007 Form in a Web Part

Using InfoPath E-mail Forms

InfoPath 2007 Focuses on Developers

InfoPath Developer Portal

Plan Forms Services

InfoPath Forms for Workflows

Architecture of Microsoft Office InfoPath 2003

Microsoft InfoPath 2003 Decision Tree: Comparing InfoPath Forms to Other Microsoft Solutions

Feedback from a reader suggested I add this, and I whole-heartedly agree:

Enterprise Solutions: A Developers Cookbook with Practical Solutions