Some of the feats in D&D 5e are seemingly less useful or otherwise not always as reliable relative to the other feats, although they are very flavorful. Keen Mind is one such feat, granting your character a 30-day eidetic memory, master navigator, and time keeper.
However, there can be workarounds for the last two benefits (benevolent DM, hand waving, finding NPCs who know the area, etc), and the first benefit does not seem to have an everyday usefulness to shortlist this feat for a wizard — not over War Caster, Resilient(Con), Alert, or Tough, for example.
At best, people seem to think it's just a way to make the DM rehash their old hints if you missed them the first time.
Here is the text for Keen Mind:
Keen Mind
You have a mind that can track time, direction, and detail with
uncanny precision. You gain the following benefits.
- Increase your Intelligence score by 1, to a maximum of 20.
- You always know which way is north.
- You always know the number of hours left before the next sunrise or sunset.
- You can accurately recall anything you have seen or heard within the past month.
If you're a wizard, you will be reading your spellbook everyday. It is within the bounds of the feat to say you recall absolutely every glyph, symbol, and diagram of your spellbook, down to the ink stains and page number of each word of each spell.
One cool way to make use of Keen Mind is to make the spellbook obsolete — within the DM's approvals/restrictions.
Here are things a wizard can do because of their spellbook:
- They can change their prepared spells after a long rest
- They can scribe new spells upon gaining Wizard levels
- They can cast rituals without expending spell slots
- They can use Arcane Recovery
- They can gain Spell Mastery or Signature Spell if they have the spellbook upon leveling up
If the wizard has the spellbook engraved in their mind, which of these things can the wizard still do, and which can they no longer do, if that same wizard did not have a spellbook at hand for some reason?
Best Answer
Things you can do, by the book: Everything you listed except scribe a new spell.
Source: PHB pg. 114
That actually requires you to write down the spell, and has a material cost associated with it that is usually associated with special inks and gems. Yes, you can recall it from memory and write it into your book, upon which you would have it memorized for a month. However the spell itself may require a specific rune written in a specific ink. Knowing that, and having written it down that way are two different things. Obviously this applies to material component cost spells only.
Since the Wizard can already replace a destroyed spell book with the spells he has prepared for the day, it's reasonable to extrapolate that with photographic memory (as this feat entails) you could replace the entire book. After all, the spells don't disappear from your mind when you cast them, so this feat should serve as a good safeguard against losing your spellbook.
Of note: You're going to want to review your book monthly and update it with new spells in order to continuously preserve this.
Of secondary note: I would even allow a Wizard with this feat who passed a successful Arcana check to experiment with and replicate a spell cast by somebody he was watching. Personal DC levels would be, DC +5 per component needed (Verbal, Somatic and Material), and double the result if the Wizard isn't proficient in Arcana. But that's a house rule thing because this is a clever use of a versatile feat that effectively duplicates the spell book.
But bottom line is: Nothing under Wizard or the Spell Book sections in the PHB prevent you from recalling everything in your book from memory. The pertinent line is here:
Since Keen Mind already ensures you have the spells memorized, there`s no reason you would need to open the book in order to review them.