The following books all mention POPFile:

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There's a good description of POPFile in Degunking Your Email, Spam, and Viruses

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Fighting Spam for Dummies

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Anti-Spam Tool Kit

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The Healthy PC: Preventive Care and Home Remedies for Your Computer

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Slamming Spam

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Open Source Solutions for Small Business Problems

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Privacy Tactics

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Canning Spam : You've Got Mail (That You Don't Want)

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PC Magazine Windows XP Speed Solutions

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The Art of UNIX Programming

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Hierarchical Modeling and Analysis for Spatial Data (Monographs on Statistics and Applied Probability)

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Master VISUALLY Optimizing PC Performance

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Information Security Illuminated