I am about to get a book printed and the term: "pdf print ready" has come up. How is this different from an ordinary pdf file?
"Print ready" is a term of art used in the printing industry to describe a PDF that has been correctly prepared so that it can be printed by a commercial printing company.
The exact requirements for a PDF to be "print ready" depend on the specific printing company that will be printing the PDF. Their printing equipment and technological setup determine what standards a PDF must meet in order for it to be deemed "print ready" by that company.
That being said, there are some "print ready" standards that are well nigh universal:
Fonts are embedded into the PDF.
Colors in the document use CMYK color space rather than RGB.
The color black in the document is the correct kind of black (e.g., CMYK black/100K black, Photoshop black, rich cool black, rich warm black, registration black).
Images are embedded into the PDF.
Images are JPG, TIFF, or EPS format.
Resolution of images is 300dpi or higher.
Line weight of hairline rules is not less than 0.25pt.
The page size (a.k.a. trim size) of the PDF pages is correct.
Crop marks are present if they are required by the printing company.
The filename of the PDF meets a specified convention (if required to).
If you visit the website for a printing company, you will probably be able to find without looking too hard a document they make available describing their "printing standards" or "file standards" or "printing guidelines." That document will probably include most of the bulleted information above plus anything else they require.