I haven't tried literarymarketplace.com, but I've seen it highly recommended in lots of books on writing. I have used Writersmarket.com (note NOT Writersmarketplace.com*), but I have often found their information to be wrong. Some agents and publishers didn't keep their information up to date and others didn't even know they had a listing there. Still, much of their information is correct. The only way to know for sure is to call the agent or publisher (talk to a receptionist!) and verify the information.
Another good source is PublishersMarketplace.com. It costs $15 per month, but the New Deals page alone is worth it.
I've been reading that the Literary Market Place is an incredibly helpful reference guide for authors getting ready to self-publish.
However… it is also incredibly expensive! Around $300.
As it's mid-August and more than halfway into 2014 already, and I'm not sure I want to buy the 2014 LMP. I've found used copies online for around $50, though they're the 2013 Edition. I'm thinking I should just buy the 2013 LMP. Any advice? I realize the market is a quickly changing place, but on a limited budget at least I'd have a baseline for reference, albeit a little outdated. That's my thought, but I could be way off.
I could also reference the book at the library, but I think I will want to reference it too often for convenience.