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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
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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.
Java Concurrency in Practice
The reference for concurrency in Java, here is note from 2015 by the author about the pertinence of this 10+ years old book.
Refactoring by Martin Fowler
To learn more about Refactoring, the natural starting point is the Refactoring Book. You should also have a look at the online catalog of refactorings
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