this is the Bible for all writers, artists, poets and others if they're based in UK. It lists all the Publishers, Literary Agents, Media outlets and lots of other useful info. I've been using it to research Literary Agents who are open to my genre of Self Help, Spirituality, Health & Wellbeing and so forth.
Having a Literary Agent is essential for getting published as the great majority of Publishers don't accept unsolicited manuscripts these days, with the exception of a small handful. Literary Agents can help with your Book Proposal - your Advert if you like - and they also find the right Publisher for you and who also offer the best deal, as in Advance Payments.
With Hay House I don't strictly need a Lit Agent as they are one of the small handful of Publishers where I can submit my Proposal unsolicited as I'm a Community Member, but having one wouldn't hurt either. Also I'm not putting all my eggs in one basket, I'll be using every option open to me to get Published, including Self-Publishing if I have to.
That's it for now,