I have published several research papers, from my PhD thesis, in varied journals and proceedings of international conferences. Now I wish to publish a book that shall be utilising the majority of the contents of these papers. Would it be called plagiarism?
If your book is simply reprinting your papers it certainly isn't plagiarism. However, if you are re-writing and re-phrasing them, to bring them together as a whole, you should do it just as if you were writing a book based on the work of others.
In other words, when you quote or paraphrase your old papers, give a citation, just as if it were someone else's paper. This makes your intent clear to the reader. More importantly, however, is that it lets the reader go back to your originals for any context that they feel they might want.
Moreover, if your book contains new material as well as the old, the citations should make it clear what is new and what was previously written.
But just restating what you said before without citation is, today, considered self plagiarism. In some circles it is considered very serious, but mostly as it prevents the reader from finding that original context along with your references and such.
Even a statement at the beginning of your book that says that you will be copying and/or paraphrasing without citation would not be appreciated by your readers.
Treat your own work like you would that of any other scholar. This is true in general, not just for the example of a book.