You must not assume that an organisation is legitimate, professional or trustworthy just because it appears in The Writer’s Handbook, Writer’s Market, the Writers’ and Artists’ Yearbook, or any other similar guide.
All it means is that the company concerned has submitted a listing for inclusion in the guide. There is no screening process involved in compiling the entries: the guides are produced by issuing invitations to every company listed as a publisher or agent in the United Kingdom, and then collating the replies.
Page 16 of the 2008 edition of the The Writer’s Handbook contains listings for Austin & Macauley Publishers, and AuthorHouse UK, which are both widely agreed to be vanity publishers; while the 2008 edition of Writer’s Market UK carries a listing on pages 287-8 for Pegasus Elliot MacKenzie Publishers Ltd., another acknowledged vanity press. If I had to hand a copy of the Writers’ and Artists’ Yearbook, I'm sure I could find a vanity publisher listed there, too.
This doesn’t mean that such guides are useless: far from it. I find them invaluable. But they must only be considered a starting-point for your research, and not as an ultimate authority.
(For those of you who don't already know, I post at Absolute Write under the user-name of Old Hack. I was threatened with legal action by Austin & Macauley a few months ago, but nothing ever came of it.)