Data Model
"The Data Model Resource Book, Revised Edition, Volumes 1 & 2"
To help us provide a strong data model foundation for the project we are using entity designs from these books and from various standards bodies. These books have been an excellent reference and useful design guide for handling business related data. Open For Business uses a variation on the models in the book, but most concepts from the book are actively used, enough so that it is very worth it to buy these books. 
Accounting and Supply Chain
"Accounting Best Practices"
This book is one of many accounting and financial management books by Steven M. Bragg. It covers general processes and methods for financial and to some extent general enterprise resource management. It is understandable for those who are not accountants, yet still of incredible value to those who are. These are some of the best practices approaches that we have already or plan to incorporate into the OFBiz applications.
"Supply Chain Strategy"
This book is to Supply Chain Management what Steven Bragg's book is to Accounting. It is a great resource for inventory and warehouse management best practices and includes numerous examples of statistical analysis in addition to various processes and methods.
Reporting, Warehousing and Mining
"The Data Warehouse Toolkit, Second Edition"
This book covers general data warehousing principles, but perhaps even more useful it also includes discussion of warehousing strategies and models for things such as retail sales, inventory, procurement, order management, CRM, accounting, human resources, financial services, utilities, transportation, education, health care, ecommerce, and insurance. In essence there is a lot of useful information here for future expansion of OFBiz.
"Data Mining, Second Edition" and "Document Warehousing and Text Mining"
Data Mining, Second Edition covers general data mining principles with practical demonstrations based on the open source Weka software. These concepts and tools are used for things such as classification, clustering, decision trees, etc. Has brief coverage of text mining, but on that topic Document Warehousing and Text Mining is much more complete and applicable. 
User Interfaces and HTML/CSS
"Don't Make Me Think!"
This is a great book about designing web sites and web-based user interfaces for ease of use and navigation. Very highly recommended, with solutions to common problems that come up in web site and applications all the time.
"The Design Of Sites"
This book has less general theory and more specific examples of good designs for all sorts of general purposes from information layout to navigation to general and advanced ecommerce.
"The Zen of CSS Design"
This is a great book about effective use of CSS for web sites. These patterns help achieve goals such as easily changing L&F with only CSS changes, and minimizing the explicit styling information in HTML, ie using more implicit structure information and special purpose classes or ids. The companion web site has some great examples: http://www.csszengarden.com/
Java & J2EE
"Head First Java"
This book is a good introduction to Java and object oriented programming principles. Recommended for those doing lower level or more technical development or customization of OFBiz, or who wish to have a better general understanding of how the system fits together.
"Core J2EE Patterns"
This book is an excellent introduction to general best practice J2EE design patterns. Many of the concepts used in the OFBiz core framework are explained in this book. It covers the various tiers of a J2EE application included presentation, business logic and persistence. As a side note, the patterns on The ServerSide are an excellent supplement to this material.
Other Stuff
"Understanding Open Source & Free Software Licensing"
This book contains good coverage of general issues related to open source licensing for both producers and consumers of the software. Also covers in depth the various families of open source licenses (MIT/BSD/Apache/Artistic, GPL/LGPL/Mozilla, Qt/Artistic/Creative Commons, Proprietary, Sun Community, Microsoft Shared Source).
"Pragmatic Version Control Using Subversion"
This is a good book about general project and development process enablement and support, with details and examples based on Subversion (including taking advantage of the improvements in Subversion over CVS that make certain processes so much easier).
"SQL In a Nutshell, Second Edition"
Just a good general book about SQL. A general reference based on the SQL2003 standard and covers details of SQL Server, DB2, MySQL, Oracle, and PostgreSQL.
"XSL-FO, Making XML Look Good in Print"
This book covers general information about the XSL-FO standard from W3C and has extensive example of how to achieve specific effects and such.