You may also come across others on your journey (optional quests) that aren't so story based, so this may go up a bit. If you don't come across some of the others, don't worry as you can pick them up at any time during the game or after the story is completed! See Misc for more details.
All optional quests are MISS-ABLE, if the quest giver dies. This is critical in Millfields, as the werewolves attack anyone if you do not engage them right away, and there are several NPC's with area-unlocking quests there : Animal Liberation, Hobnobbing With Hobbes, and Restoration. If even one of these quest-givers dies before you do the quest, you will not be able to complete the gnomes, keys, and books quests, as the Hobbes cave, the locked area of Bowerstone Industrial, and the island of Driftwood all contain multiple collectables.
For me it was the stupid Hobbles cave, making keys and books uncompletable. Do all these quests as soon as they unlock, and do not leave the area (save) if a quest giver is 'missing' from where they should be