I agree with NatCh's comments in general but am more skeptical that there is sufficient space in a typical image to "hide" a full book's text, especially since some books include additional images.
Also, JPEG (in general) is a lossy compression technique. There is one version that is not lossy but that is typically not used for most JPEG images and is definitely not compressed as small as the lossy ones. With images the lossyness may not be noticeable but it probably would with the text. However there are lossless image formats that could be used, e.g. GIF, TIFF.
In addition, an image file can be "artificially" increased in size with no apparent change to the image. I actually once (long ago when I could still spell C) wrote a C program that would add text to an image file in this manner. The "encryption" was along the line of "The Purloined Letter", i.e. it was really not encrypted if you knew where & how to look for it. (I was dealing with relatively ignorant & non curious people, so it worked fine to hid the "message" from them.) However any computer literate, curious person would have quickly discovered what was going on. That led me to use the UNIX "crypt" command. It is nice, in that if you attempt to unencrypt using it and use the wrong key, what you achieved was to double encrypt it.
It was all fun & games during my youth.
Now it would just be work.
