The best way to read an art book depends on the individual reader's preferences. For those who prefer the traditional feel of physical books, reading a print copy of an art book is probably going to be the best option.
Hey, you get to appreciate the artwork up close and personal, as well as experience the tactile sensation of flipping through and smelling the pages. If you prefer the convenience and portability of digital books, then reading an art book in a digital format is a great option. However this is probably better done on a tablet, iPad or desktop as a phone is really too small.
A plus for digital books is that they are often cheaper than print copies, and they are also accessible from anywhere with an internet connection.
The main difference between using books and a digital reference for art research is the availability of information. Books can provide more detailed information, but they may not always be available or up-to-date. Digital references, on the other hand, are more readily available, often containing more up-to-date information, and by and large are easier to search through for specific information.
Digital references also allow for the integration of other media, such as images, videos, and audio. They will also often have links for further searching.
A notable exception I have found is the information that can be accessed in some much older books. I have found, particularly in older books discussing the more technical details of learning to draw, information often unavailable in more modern versions. See the above illustration for a great page on how to draw handles - always a tricky aspect of still life drawing.