"Introduction to Bedside Ultrasound" is a revolutionary text that seeks to teach bedside ultrasound to students in a more efficient, directed format than previous texts. It accomplishes this with a plethora of multimedia adjuncts. Instead of simply reading about ultrasound pathology and protocols, the reader can watch actual videos and demonstrations of the pathology from directly within the book while reading. This experience is closer to bedside learning with a true expert than reading a traditional textbook."
These 2 guys are the founders of the main ultrasound podcast, very good presenters, and this (free) textbook is EXCELLENT! Highly recommended starting point for learning ultrasound for EM. It is an Ebook so there are videos imbedded in it.