According to Webster's New World College Dictionary, 4th Ed., to plagiarize means to take ideas, writings, etc. from another and pass them off as one's own.
Please note that you are contracted for original work. Original work is work created by the contracted (or subcontracted) author for any given project. If you use material that is not original work (either text or art), you must get written permission from the copyright holder of that material (even if it is Wiley); this includes text from your previous editions or works with this company.
Many writers consult program documentation, reference works, and competing titles when planning and writing a manuscript. As an author, it is your responsibility to paraphrase adequately each and every sentence. Even paraphrased material must be cited by source. If you must take information word for word, it should be in quotation marks with appropriate bibliographical citation (which your editor can help you with). Word-for-word quotations should be kept to a minimum.
Finally, note that even if you use information that is freely distributed in the public domain (such as Websites, RFCs, etc.), you are profiting from the material and must therefore receive permission. Occurrences of plagiarism are grounds for cancellation of contract.