This ebook allows limited simultaneous users. The best approach is to download, (or email to yourself), the maximum number of pages allowed, in this case 60. Please don't download the entire book at one time. When you log out of the UML library and log in again, the number of allowable pages will reset, and you can send yourself another chapter or whatever you need. You can also read the book online.
If you click on the book and find you can't get access, just wait a while and you should be able to access it.