Make sure that you use the quick turn button (R) to check what is behind you. It is hard to get into the habit of doing this if you are used to other FPS games, but pressing it once, to check behind you, or twice, to have a quick glance and return to face the front, you will make sure you do not have anything sneaking up on you.
Ammo is obviously vitally important in ZombiU as the easiest and safest way to take out roaming zombies. You should preserve your ammunition for when taking on larger groups of zombies.
A great way to take out zombies without using ammunition is to use the surroundings to your advantage. If you find a fence that zombies have to crawl under or objects they have to clamber over, stand on the other side with a baseball bat and smash some skulls!
Be aware also that you can lure the bomber zombies towards others and then blow up the gas tanks that they carry to take out additional enemies without wasting extra bullets.
Rather than using bullets on huge crowds of zombies, deploy blanket weaponry such as flares to draw in the enemy, then grenades, molotovs and mines.
The RADAR is absolutely crucial in ZombiU. You need to use it to spot where enemies are but make sure you remember that it also picks up animals! Using it to pick up any zombies in a particular area can make it much safer for you to pass through, however.
After you die in ZombiU your weapons will stay upgraded. If you die before you are able to loot your former-self as a zombie, then the weapons and items that you had will appear in sections of the game that you have previously visited.
If you scan CCTV boxes you can use the safehouse computer to spot loot that has respawned in areas that you have visited previously.
For the Raven door code in the warehouse when you are going for the second book, you first need to complete the Tower of London mission. Then, you can find parts of the code spread out in the bunker. The first is in the mirror by the door, the others are found by scanning with the blacklight in the room - follow the blood trails to get them.