Nick has a theory: the best way to make friends is to be the host. Don't wait for people to invite you to events, invite them. People actually want new friends, and everyone's pretty bad at actually organizing people together. Being the central link makes you instantly "popular" with a group of people. This is experientially true: the best way to get a group together is to organize it yourself. However, this leaves out tactics. How exactly do you do this?
2-Hour Cocktail Party is one specific, highly detailed implementation of Nick's theory. He's hosted hundreds of these parties, and refined the process to be reproducible and systematic. He covers how to handle invites (even when you don't know many people), what to do during events, and how to make sure events are successful.
Nick doesn't think this is the only way to do gatherings, but is one way that absolutely works. You'll slide past most pitfalls people fall into, and can adapt the strategy to your own situation as needed. And for this, it's a great book. Combine with The Art of Gathering for a holistic view of gatherings.
(I actually got a copy of this book last year from the author, pre-ordered a paper copy as well, and recently read through it again.)