Hi Darren -
If you don't have any experience with military medicine, I would also recommend that start looking for Army manuals. First I would start with FM 21-11 First Aid For Soldiers, FM 8-50 Bandaging and Splinting and FM 8-35 Transportation of the Sick and Wounded. I would also recommend TM 8-220 Medical Dept Soldier Handbook. The handbook covers a lot of the important stuff covered in the 3 prior FMs plus other general Soldier stuff. These are good books about how the Army wants medics to act and what they should now. As you want to expand your knowledge - or if you decide to move away from the front line medic role - look for any FM8-* to include FM 8-10 Medical Service of Field Units and FM 8-55 Reference Data. Both of these more detailed medical info. Even if you have military medicine experience, I would recommend getting the manuals as military medicine has improved in the last 60 years.
I used to have a link that you could download TM 8-220 but can't find it now. If you go to this link
you should be able to find a good selection of the above mentioned FMs ready for download in PDF.
Just checked eBay and there are several original copies of TM 8-220 available for under $10. I would consider anything under $20 a good price for the TM. Original copies of the FMs are harder to find and prices reflect that.
I would also recommend that you check this list of manuals
that Ben and Alain have already put together.