The British Institute of Radiology (BIR) has published a free e-book on the safe use of ultrasound.
Edited by Dr. Gail ter Haar, the third edition of The Safe Use of Ultrasound in Medical Diagnosis is written for the practitioner and covers basic concepts important for the safe use of ultrasound, according to the BIR. It also directs readers to extensive literature on the topic.
The e-book can be accessed here.