Check out some of the books in my library...

• Exam Cram series
• Microsoft Press Home Page (for Microsoft brand books)
• MacMillan Computer Publishing
      Que (excellent end-user books, but decent tech books also)
       New Riders
• O'Reilly
• Osborne
• Sybex
• Syngress (excellent ISA Server book, and others)
• Wrox (books for developers and scripters)
  

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• Bookpool (after you find the book you want... NO ONE  has cheaper tech books when it comes TIME TO BUY)
  
• You may also want to check HalfPriceBooks.com for cheap books. Thanks to Rey Bautista.