“A book that increases skill by 2.” —In-game description, New Mystery of the Emblem
The Secret Book (秘伝の書 Hiden no sho) is a Stat booster that recurs across the Fire Emblem Series. When used, the Secret Book permanently raises its user's Skill by a set number of points that varies from game to game.
In Genealogy of the Holy War, the Secret Book is substituted by the Skill Ring (スキルリング Sukiru ringu), an item that must be placed in the inventory of any unit in order for its Skill-boosting property to take effect. The Secret Book is similarly substituted by the Skill Ring in Thracia 776, although this particular incarnation of the ring functions as an expendable item that permanently boosts its user's Skill by 2 when used.
In TearRing Saga: Utna Heroes Saga, the Secret Book's equivalent is a potion that is simply known as the Skill Plus (SKL プラス SKL Purasu), permanently raising its user's Skill by 2 when used.
The relative utility of the Secret Book varies from playthrough to playthrough and from game to game, depending upon average enemy Evasion rates, whether or not Skills are available, and whether or not the player chose to use low-accuracy units. In games such as Fire Emblem: The Sacred Stones, where no skills are triggered by the Skill stat and few enemies possess high evade rates, Secret Books are not very helpful and might even be put to better use getting sold, but in other games they are far more useful.
If Skills are not usable, Secret Books should be given to powerful units that are burdened with poor hit rates, typically Fighters or Knights. If Skills are usable, players may want to use Secret Books to boost their chance of activation. An additional 2% chance of activating Sol could be the difference between life and death.