I have used both The Writer's Handbook AND Writers' and Artists' Yearbook for 15 years and have used them to sell 7 books including British editions of US titles I've written. Which one is better? It depends on the year. In fact, in the 1990s, The Writer's Handbook was the first to make detailed listings of book editors, the key to getting your query letter or proposal read. The Writers' & Artists' Yearbook, however, fought back and greatly improved the detail of its listings, surpassing The Writer's Handbook for a year or two. Now it's a see-saw battle to provide ever more information and keep up to date: The writer is the beneficiary of this competition. For my money, buying both books is worthwhile, just to compare the listings and to get the fullest picture. You only have one chance per idea with a publishing house. Once an editor turns down your idea, you can't resubmit, so you want to make your best shot the first time.
-- Erich Hoyt (author, The Earth Dwellers, Creatures of the Deep, Insect Lives, Orca: The Whale Called Killer, and other books)