How do DPI and PPI affect my book project?

The terms DPI and PPI have evolved into meaning the same thing, but in actuality, the terms are as different as a printed catalog and a web page.

DPI (Dots per Inch)  was the common term used in the pre-computer era of printing  onto paper.

The more dots per inch (a higher DPI) the higher the printing quality.

PPI (Pixels per Inch) is the term for the new digital age of graphic reproduction.  Scanners, digital cameras, and digital print devices use PPI to measure  image quality. Images are measured in PPI.

Images are measured in PPI so what does this mean to you when making your book.

To understand PPI, let’s try an example:

We scan a 1-inch square photo at 200 PPI and place it into Quark or InDesign (professional page layout programs) and resize it to 50%. The Pixels per Inch are now closer together when the image is at 50%, so the PPI is now 400 PPI. If we take that original 1-inch 200 PPI image and resize it to 200%, we are spreading the Pixels per Inch further apart so the PPI is now 100 PPI.