Loot the content of all 5 chests. The first chest is easy since it is on your right when you enter the ruins. The second chest is guarded by a terror on your left. I use the TBM steps trick to loot that chest. If the terror notices the party, it's time for reload since exiting and reentering the ruins wont help - the terror will be already waiting for you. The third chest is easy - swim underwater past all the terrors and loot the chest which is also underwater. The fourth chest really is not necessary and it is also very hard to loot. There are at least 3 zombies guarding it and the chest is quite far from the entrance so the TBM trick does not really work. Since zombies are a lot smarter than terrors and CAN open the doors themselves, you have to kill them which is not easy at all. I must admit that I often simply skip this chest. The 5th chest is nearby the Silver Book so that's where you'll eventually come if you are after that book.

The final door requires a key. To grab the key from that room which is guarded by terrors all you need is to use TBM trick. Even if the terrors notice you, if you manage to close the doors, they aren't able to open them. Wait some time to calm them down and save. Run past the skeletons with TBM steps.

Now I use TBM trick to get in that room which is guarded by a single zombie. I use TBM because there's that terror watching for that area you have to cross. If you do not managed to sneak past it, it will be quite hard if not impossible to get out from these ruins. Now, kill the zombie (which isn't very easy at all) and rest to recover your HPs. Also, I wouldn't advise to fight against two zombies at the same time because they always enjoy resurrecting each other. Now proceed to the next room which also is guarded by one zombie. Kill it and using the TBM trick loot the 5th chest (watch out, there's another terror), take the book AND close the doors. Now all you have to do is RUN! Make sure nobody is dead when you are outside.