I'm playing a 6th-level Tiefling Warlock in D&D 5e. On a recent journey, I acquired a spell book that has something to do with a slithering trapper. How do I add this to my list of spells?

Warlocks of the Tome can learn rituals from spellbooks. Other Warlocks cannot use a spellbook for anything useful; you don't learn spells from spellbooks, you learn them from your Patron.

For this particular spellbook:

Nothing in that spellbook is useful to a Warlock

This particular spellbook contains:

(spells of an Enchanter and the ritual to create a slithering tracker)

The ritual to create the tracker is a bad idea to cast; it has no (non-plot) value.

It involves, among other things, sacrificing a life:

"The ritual is evil magic. Adventurers know that an attempt to cast or copy it is ill advised."

The exact consequences of casting the ritual are in the module.

Trust me, you don't want to do it. Treat it as uncastable and something you should destroy.

The spells in the spellbook are:

1st level: charm person, mage armor, magic missile

2nd level: hold person, invisibility, suggestion

3rd level: fireball, haste, tongues

4th level: dominate beast, stoneskin

5th level: hold monster

I am unaware of any of them being ritual spells.

(Source: Murder at the Stop adventure)

Spell books may have some cash value. This is not determined in the D&D books, however. Here is an attempt at calculating it.

They can't, generally

Warlocks don't learn spells from spellbooks, they learn them by leveling and they are granted by your patron. You could multiclass into wizard to learn spells from spellbooks, but that's not a very good option in terms of power.

However, you might still be able to gain something out of the spellbook if they are rituals.

If you've picked up the Pact of the Tome feature, you can get the Book of Ancient Secrets invocation. This will let you:

On your adventures, you can add other ritual spells to your Book of Shadows. When you find such a spell, you can add it to the book if the spell's level is equal to or less than half your warlock level (rounded up) and if you can spare the time to transcribe the spell. For each level of the spell, the transcription process takes 2 hours and costs 50 gp for the rare inks needed to inscribe it.

So if the spellbook you found has any ritual spells, you'll be able to add them to your Book of Shadows if you have the Book of Ancient Secrets invocation.

As a Warlock, you learn your spells through your Pact Magic which is granted to you by your Otherworldly Patron.

Learning spells from spellbooks is a wizard's class feature (PHB 114).

As a Warlock, you learn your spells through your Pact Magic (PHB 107) which is granted to you by your Otherworldly Patron (PHB 107). You learn new spells by advancing your character level and you only have accsess to spells from the warlock's spell list. As a warlock you don't require a spellbook.

At 1st level, you know two 1st-level spells of your choice from the
warlock spell list. The Spells Known column of the Warlock table shows
when you learn more warlock spells of your choice of 1st level and
higher. A spell you choose must be of a level no higher than what’s
shown in the table's Slot Level column for your level. When you reach
6th level, for example, you learn a new warlock spell, which can be
1st, 2nd, or 3rd level.

Additionally, when you gain a level in this class, you can choose one
of the warlock spells you know and replace it with another spell from
the warlock spell list, which also must be of a level for which you
have spell slots.

Warlocks who have the Pact of the Tome (PHB 108) have access to a Book of Shadows which is not a spellbook.

When you are at 2nd Warlock-level, and you are a Pact of the Tome warlock, you can choose the Eldritch Innvocation: Book of Ancient Secrets (PHB 110):

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 spells appear in the book and don’t count against the number
of spells you know. 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. You can also
cast a warlock spell you know as a ritual if it has the ritual tag. On
your adventures, you can add other ritual spells to your Book of
Shadows. When you find such a spell, you can add it to the book if the
spell’s level is equal to or less than half your warlock level
(rounded up) and if you can spare the time to transcribe the spell.
For each level of the spell, the transcription process takes 2 hours
and costs 50 gp for the rare inks needed to inscribe it.

Which allows you to inscribe ritual spells from any classes' spell list (Specific Beats General; PHB 7) into your Book of Shadows, that is spells with the ritual tag. If you can read the spellbook and are of high enough level, you then can spend 2 hours and 50 gp per spell-level to inscribe the ritual into your Book of Shadows.

The ritual to cast a Slithering Tracker (VGtM 191) is not a spell with the ritual tag from any classes' spell list. It is also not on any of the Hags' spell lists (MM 176-179), nor on the Lich's spell list (MM 202) and Priests of the God of Vengeance have no stat block.

But your Otherworldy Patron might grant it to you (if your DM so decides), ask your DM whether you will be able to transcribe the ritual for Vengeance at Any Cost (VGtM 191) into your Book of Shadows and how much that will cost you, as there is no defined spell outside of the ritual information provided on VGtM page 191 (there is no spell-level that makes the cost predictable because there is no description-block for the ritual).

Warlocks cannot learn spells from spellbooks

Without multi-classing as Wizard, that is.

Among the spellcasters of DnD 5e, there are four categories:

• Innate casters (primarily Bards, Rangers and Sorcerers) learn spells upon leveling up. They know relatively few spells, but don't need to prepare them to use them.


• Divine casters (primarily Clerics, Druids and Paladins) do not learn spells --- instead, they can prepare spells from the entire spell list available to their class.


• Wizards only know their cantrips permanently --- the rest of their spells are in spellbooks. They tend to have lots of spells because in addition to adding new spells to their book upon leveling up, they can copy spells from scrolls and books they find. However, they can only prepare a limited number of those spells each day, and can lose access to spells if they lose their spellbook.


• Warlocks are not true spellcasters but instead know Pact Magic --- they know their spells innately and gain new ones through leveling up like innate casters, but have fewer slots but regain them during short rest.

Spell books are specific to a Wizard's way of learning and using magic --- a Warlock cannot, by the rules, benefit from one apart from possibly selling it, or gifting it to a friendly wizard.

Warlocks can have access to a Book of Shadows if they choose the Pact of the Tome, but it is not a "true" spellbook nor grants access to them --- and you wouldn't find one lying around, but receive it from your character's patron. However, if they choose the Book of Ancient Secrets Eldritch Invocation, they can copy ritual spells into their Book of Shadows; however, they can be only cast as rituals.