The legality of the book is based on copyright and the license under which it was released. Yes, if released under a Creative Commons license of the appropriate type. If you employ the phrase "all rights reserved," you may be placing yourself in danger. Copyright law allows for certain activities called "fair use" that allow for limited copying without violating copyright law. For example, if you want to quote small portions of the text in an academic setting, you can do so without asking permission from the publisher or author. You are allowed to do this because it is considered fair use. There are four factors that courts look at when determining whether something is fair use: 1 the purpose of the use; 2 the nature of the work; 3 the amount used; 4 the effect of the use on the market for the work. So in conclusion, reading books on podcasts is not only acceptable but also legal as long as you include proper citations.