I thought Ridiculously Simple was awesome... of course you can't use that alone, but if you devote one weekend to it before starting your clerkships, it'll give you a great general overview of most of the basic stuff you're likely to see... then you can use another Rads source for the details. Good luck.
Brandt Helms is a very good book, but it is an oversimplification in areas (Bone) and down right painful in others (Chest). It also is not cheap. I think that it is a good investment for first year residents, but not for med students. Harris is overkill for a MS and some residents, but is an excellent text to prepare for call. The absolute crucial learning for a med student are
2. Clinical medicine.
3. Radiology basics such as normal vs abnormal and what studies are best for detecting specific pathology.
Ridiculously simple is a cheap book that will give you some basics and has few black pearls (Misinformation). Squires is an excellent text but I think it is about $80 or so. Three books that I think are outstanding for students and will get a lot of use in the first year of residency are
1. Imaging atlas of Human Anatomy by Weir ($40) versus Anatomy in diagnostic Imaging by Fleckenstein ($120)- better but probably not worth the extra dough). You can never learn enough Anatomy, and the more you know; the smarter you look. Also, you cannot effectively recognize abnormal until you know normal and normal comes in many varieties. The biggest mistake non-radiologists and occasionally even experienced radiologists make is misinterpreting a normal variant as abnormal Which can result in unnecessary and sometimes devastating results to the patients. Many kids have lost extremities for benign bone lesions such as myositis ossificans surrounding a previous pelvic avulsion fracture.
2. Radiology Recall by Spencer Gay and woodcock (PD and previous chief residents at UVA). No images just drawings. Gives introductory basic anatomy in each section with drawings. I like that they give lots of differential lists. I really like this book, but I reccomend taking it to kinkos immediatelyt after purchase and have it bound (Wire coiled) because the pages will fall out. ($30)
3. Radiology Secrets. One of my favorites. Gives the down and dirty facts with decent images. No anatomy or differentials. In the same Question/Answer format as others in the series. I like it because it can allow you to cover most major points of a subject such as pediatric bone disorders rapidly and can make a med student shine (Hint: Never outshine the residents . ($40)
Just my $0.02 - hope it helps. Oh yeah, when buying rads books checkout Walmart.com. They are usually the cheapest.