Thank you for contacting the library regarding the Roitt's Essential Immunology e-book.  This book is part of an e-book collection purchased by the entire CU system libraries.  Unfortunately there is a limit on how many users can open the book at the same time, and you probably saw a message saying that the book is being used.  You may want to come back some other time.  If you are seeing other kinds of error messages, please let us know and we will work with you to troubleshoot the problem.
Thanks! 
Yumin Jiang
Head, Collection Management