This pdf includes a new Warlock patron, the Empowering Cult. It also includes a new boon, Pact of the Chalice, 4 new invocations for that pact, 3 new spells, and a sample character. The new patron is a nice twist on the concept of the warlock -- you have a cult (of at least 1 person, but it can go into the thousands) that empowers you from their belief. The tradeoff for the power you get is that your believers want you to fulfil their wishes, so I cult that starts wrapped around your finger can get out of your control as you get more powerful. So it's different than the usual warlock patron, but it can provide some very interesting stories if your interests and the interests of your cult start to diverge.
The Pact of the Chalice is an interesting variant, but I wouldn't be able to resist turning my warlock into Lil John and screaming "yeah!" all the time as I flung the contents of my Chalice on someone to cast Eldritch Blast.
The three spells include Arcane Convention, which allows multiple casters to cast a spell together which a much longer casting time than usual; Arcane Gift that lends a spell or mystic arcanum to another creature; and Arcane Pact, which give another creature a level of warlock. So, we have a cult casting a spell together, like in any good cult story and two spells based around the idea of a high-level warlock becoming a patron of other warlocks themselves. I haven't run any of these spells in a game, but they don't have any obvious game breaking effects.
Is this book worth getting? If you like running or playing warlocks, I'd say it's a solid yes. Cults make for excellent evil opposition for PCs, but the idea of having a good warlock with a cult is intriguing and this supplement would help you pull it off. If you GM sword & sorcery stories, get this. If you play warlocks, I'd also recommend this for variety.