I do not think examkracker's books are too brief. Relative to other companies, they do have significantly less material to cover in their texts. The reason for this is, I believe, two-fold. The information that is given in the texts is presented in a very pithy and very well-thought out manner. This results in a conciseness that reduces the verbosity of the text. Secondly, the authors really seem to have researched thoroughly what can be expected of AAMC's exams...and as a result have reduced the topics and their depths accordingly.
Both of these factors contribute to review materials that are considerably shorter than what their competitors produce.
I'm not saying that the ek books don't require at least a small amount of background knowledge before using, because they do in parts. However, this knowledge really is most elementary. If I've run into a misunderstanding in examkracker's text, I simply look to the textbooks I used in my undergrad classes.