The Tome of Great Value is a quest provided by Ammon, in which you have to bring her some Cobalt Moss to complete a potion she is creating. When you deliver it she will ask you whether or not you could help her get into Candlekeep for which you can sign a recommendation, if you wish.
Possible Outcomes Edit
If the main character is a Mage or Sorcerer, there is an additional dialog option in which you can offer to concoct a substitute for the Cobalt Moss, allowing you to complete the quest without going into the dungeon.
Retrieving the Cobalt Moss and asking for "appropriate compensation" will grant you 50 gold during the conversation. This is in addition to the reward(s) you will receive after the conversation and quest are completed. There is also an option to give her the item without requesting compensation.
Regardless of which method is used to complete the potion, once finished, Ammon will divulge that she intends to use it on a tome at Candlekeep, but has not had success finding a rare and valuable tome with which she may gain access.
If you do not mention having been raised at Candlekeep or refuse to sign a letter explaining the situation on her behalf, the quest will be completed and you are awarded 50 gold.
If you agree to sign the letter, in addition to the 50 gold, you are also awarded 3,000 EXP and a Wand of Monster Summoning.