Be sure you downloaded the version of Book that works with your version of Moodle (download from here).
After downloading the zip file, upload it to your moodle site using an ftp program. I typically upload to my home directory.
After you have uploaded, unzip the file and move the entire structure (rooted at book) into the moodle mod directory.
If you have shell access to your site, you can do the operations in the last paragraph from the command line. Otherwise, use the File Manager on your Web host's control panel.
This video shows how to upload and unzip a mod in cpanel. I used the legacy file manger for the screen cast but I think that is available on your server http://www.e-blackboard.org/mod/resource/view.php?id=509 The mod I demoed was actually skype but book works the same way. Once the install is complete you will find book (when your editing mode is turned on) under resources.