It appears that there are 45 investigative agencies in the 2009 AT&T book. Of those, 4 was not in the book last year. Of those 4, three have actual ads. 1 of the more impressive ads in the book is from a company that did not have a listing 2 years ago.
Moral of the story - Ask an agency how long they have been licensed before you hire them. You don't want to give a company a retainer, and then find out they just got licensed.
I strongly recommend that you check out multiple years of phone books to see who has been around. That applies to what ever area you are in. The private investigations business is extremely high turn over. You don't want a fly by night agency.
Next I need to find a 2007 book (I have one around here somewhere), and find out how many of the current companies, was not in it.