Book of Ancient Secrets

You can now inscribe additional magical rituals in your Book of Shadows as described below. The spells 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.

Taking this feat does not confer any new spells in your Book of Shadows by itself. But 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.

If the Book of Shadows is conjured again (so that the old one is destroyed) it still contains the spells that you have inscribed in it. They are not lost.

The 5e Warlock could take Pact of the Tome Pact Boon at 3rd level, giving the warlock a Book of Shadows containing three cantrips from any class. At 3rd or higher level the warlock could take the Book of Ancient Secrets invocation to gain two 1st level Ritual spells of any class in the Book of Shadows, and the ability to inscribe additional Ritual spells as described above.

For the 5.5e Warlock, Pact of the Tome is now an invocation, and the Book of Shadows contains three cantrips and two 1st level Ritual spells. There is no way to inscribe additional Ritual spells into the book. This is a loss of a potent utility (and roleplaying) feature for warlocks. Since Pact of the Tome now costs an invocation and in 5e Book of Ancient Secrets cost an invocation, if your DM allows, you can take this homebrew Book of Ancient Secrets feat for free at 3rd level or higher provided you have the Pact of the Tome invocation. (The current homebrew builder doesn't allow enforcing Pact of the Tome as a prerequisite.)

If you swap out Pact of the Tome for a different invocation, you lose the Book of Ancient Secrets feat also and your Book of Shadows is destroyed. If you subsequently take the Pact of the Tome invocation again (and are at least a 3rd level Warlock), you can again take the Book of Ancient Secrets homebrew feat for free, but your previous Book of Shadows and all of the Ritual spells you inscribed in it is lost. You will be starting the Book of Shadows over from scratch.

Note that the current homebrew builder restricts the Ritual spells available for you to select to spells of a level less than or equal to half your total character level, rounded up. The intention of this feat is to restrict the Ritual spells available to spells of a level less than or equal to half your Warlock level, rounded up. However the homebrew builder doesn't allow that condition to be expressed on a feat. So please honour the intention even though it can't be enforced.