Writing a new book on known methods/algorithms
Plagiarism is literally copying the text (or making only minor changes to pretend that you are not copying the text literally). Writing a book about something that is known is not plagiarism by itself, as long as you use your own words to explain the "standard methods".

If you do this, and if you do not copy figures and exercises (if there are any) it is not plagiarism. If you do need to copy a figure you usually need permission from the publisher (because of copyright, but that is a different issue than plagiarism).

So if your book has a different structure, is written in your own words, and cites the sources, it is probably not plagiarism. Your text simply needs to be clearly different from the original text(s).