A Bard in my group has expressed an interest in attempting to read a Warlock party member's Book of Shadows, and I'm trying to figure out what, if anything, they can get from it. I figure they can't actually cast anything with it, since it's not made of spell scrolls, but they might be able to identify some of the specific spells in it.

I know some related RAW rulings, but I'm not sure if they apply to this case. For example, this related answer says that a caster can make sense of a spell scroll if the spell is on their class's spell list, but as previously stated, this isn't a spell scroll.

So, my guess is that one of these is the case, but I don't know which one: the Bard reading the Book of Shadows

• cannot read it at all


• can identify that it contains magic spells but no more


• can identify those cantrips and spells that are on the Bard spell list


• can identify any spell in the book, perhaps with a successful Arcana check

Which is the case? Or is it something different?

(I assume the answer would be equally applicable to any other spellcasting class except possibly Wizards.)

Legally, the book is useless to the Bard. How that manifests lore-wise is up to the DM

A Book of Shadow only contains cantrips:

Your patron gives you a grimoire called a Book of Shadows. When you gain this feature, choose three cantrips from any class's spell list.

If that Warlock also has the Book of Ancient Secrets Invocation, they may also add Rituals to the book:

You can now inscribe magical rituals in your Book of Shadows. Choose two 1st-level spells that have the ritual tag from any class's spell list (the two needn't be from the same list).

Given the fact that a class may only read spells from scrolls, etc that are in their class list, this would mean that a Bard, at best, could only read rituals and cantrips which area already on the Bard spell list.

However, the bard would not be able to make use of any Bard spells in the book because a Bard is limited in the number of cantrips and spells they know (as are several other spellcasting classes).

On the other hand, A DM might treat the grimoire as a spell scroll and allow the Bard to cast the book's contents while it was on her person. But this would then mean the Warlock could not then cast those spells because the pact also says:

While the book is on your person, you can cast those cantrips at will.

And the Invocation: With your Book of Shadows in hand, you can cast the chosen spells as rituals. You can't cast the spells except as rituals, unless you've learned them by some other means.

This means that, without the book, those spells are not castable. Once the Bard handed the grimoire back to the Warlock, the Bard would then lose access to those spells. The net number of cantrips & rituals known in the party would remain constant either way (unless the Warlock knew any of the book's ritual spells innately.

How the DM decides to handle the mechanics from a lore standpoint is their choice. The Bard can identify the type of magic for each spell (this is in keeping with the rules of Detect Magic spell) but that's of no real value. The PHB offers this lore explanation:

Or [your Book of Shadows] could be the tattered diary of a lunatic driven mad by contact with the Great Old One, holding scraps of spells that only your own burgeoning insanity allows you to understand and cast.

But maybe the Bard is unable to read the book's writing at all because the text appears as swirling, formless shadows, etc, so the nature of the magic is all she can glean.

There are lots of options for how to explain this limitation.

The Bard gains inspiration

“What, if anything, they can get from it” is, at minimum, a point of inspiration.

plundered magical knowledge from a wide spectrum of disciplines

...taken straight out of the description for Magical Secrets, is an indicator that the type of curiousity displayed by the Bard in your party is good roleplaying, and it should be rewarded.

The rules on using inspiration state:

Additionally, if you have inspiration, you can reward another player
for good roleplaying, clever thinking, or simply doing something
exciting in the game. When another player character does something
that really contributes to the story in a fun and interesting way, you
can give up your inspiration to give that character inspiration.

I would not rule that the Book of Shadows functions the same as a spell scroll, because if a Book of Shadows could be used as a scroll, why not a wizard’s spellbook? Spell scrolls are not merely instructions for casting a spell. They are the embodiment of a spell. The fact that they crumble to ash after use speaks to this.

But given that digging for magical secrets is within a bard’s purview, and we are talking about a literal Book of Ancient Secrets, a DM might choose to let a Bard read and identify spells contained in a Book of Secrets with an Intelligence (Arcana) check, similar to how a spell scrolls are deciphered.