Wanderer Scrolls
 

I have 5 wanderer scrolls. Can they be used more than once, or should I keep them for an emergency?

Always keep them for later use. Ancient Knowledge must be used in the last battles (including the boss battles), where the XP is the most so that you can get the most out of them. +10 Atk/Def, +30 rage/mana scrolls are very useful in boss battles and castle sieges.

Scrolls like Song of the Wind, Death Call, Call Collosus will give you a full troop, so it's up to you to decide when do you need some extra firepower.

They get used up once used but you will get a reasonable amount from duplicate Navigation Charts. More if you use the Dragon's Treasure Searcher ability (if you're lucky). One of the companions can also make a random scroll once every 30 battles which is quite nice.

I like to use some just before I want to hit a couple of Lethal or Invincible Hero fights where the unit makeup looks tricky for my current forces / strategy. Attack, Defence, Intellect, Rage and Mana Scrolls last for 2 combats. Crit scroll lasts for 3 combats. Experience one lasts for a whoping 5 combats (don't waste it on weaker or equal strength enemies - best used against Lethal / Invincible enemies lead by a hero). The leadership one only lasts for one combat so if you use it to make a stronger army for a particularly nasty fight and you don't take heavy losses don't forget to offload the excess troops somewhere before the next combat. If your entire force is over your leadership limit you automatically lose when the fight starts (I tried it once just to see : ) )

If money is an issue the ones that summon units are a great way to get a full stack at next to zero cost.

They also sell for a few thousand gold so if you don't want to use a particular type (usually one of the three types of unit summon scrolls) it's a nice way to get a bit of extra cash.