Hi, you sound like a keen conscientious student. Good show! Just because something is not necessary does not mean it will not do any good. My first comment would be that the course structure and its administration varies from uni to uni. So it is difficult to suggest any specific books as the subjects done early on will vary somewhat.
Having said that, a basic understanding of the core principles would be interesting reading for you, and it would give you a slight headstart and insight into your chosen career. Medicine is a gruelling course, and any such advantage can only help.
I would not spend too much money on books r n though [medical books are not cheap]. Also, a lot of students prefer to do some/most of their studying online. If you are a books fan like myself, I would suggest getting a couple of smallish broad-based books on a couple of topics that interest you generally.
Use a website like
www.alibris.co.uk [also compare with the infamous amazon!] where you can pick up super bargains of just slightly outdated editions, but if you are anything like myself [I just cannot read a book that has its corners carelessly bent OR its pages ruined by someone who has licked their fingers to turn the pages [ugh!], wait stealthily like a cheetah before pouncing, when a "Like New" copy comes up from reputable companies like "AnyBook" with a great feedback record, where sometimes you can pick a very well looked after copy for next to nothing (at this early stage, you do not need to know the latest research that drives clinical decision making, so a slightly older edition is well good enough).
Some core subjects that most unis are likely to take up early on are [as you guessed] anatomy, physiology and histology. I would not make things too hard r n so think about the "An Illustrated Text". series OR "At A Glance" series [most of which are around 100 pages], and get 1-2 to get a feel.
Another thing to think about is something like the excellent {though v old) series of novels on medical issues (v nicely leather bound copies btw) by Reader's Digest, which make a combination of medicine and a novel-like story to make things a bit more, shall we say, palatable.
Hope this helps!
M