[1-10-2019: The Anesthesia Trauma and Critical Care e-book is no longer available online.]
The 8th edition of the trauma course manual, Anesthesia Trauma and Critical Care, is available as a free PDF download.
I keep a copy of the book on my laptop, tablet and phone.
Dr. Weingart of EMCrit says that this manual is even better than the Advanced Trauma Life Support (ATLS) manual. And the ATLS manual is not available as a free online download.