The yearbook is good for general reference and maybe as a means of identifying new agents/publishers you might not otherwise be aware. But the way these reference books are put together means the information contained is likely to be many months old by the time the book is published. However that doesn't mean to say lists on the internet are kept any more up to date.
I am in the process of approaching agents for a travel narrative. I have used older versions of the Yearbook, plus online sites such as
Bloomsbury to identify agents.
I then do as much internet research as possible, using Google to search on the name of the agent to find out if they have a website (quite a few have different or more up-to-date submissions guidelines than those written in the reference books). You might also be able to extrapolate a website from an agent's email address.
I also do searches on any names of authors that pop up during this research, to see if they write books similar to my own and then track them down on Amazon to see what publishers the agent deals with.
I think it's worth putting the time and effort into doing your own research into every agent you intend to submit to in order to weed out the ones it is pointless spending the postage on.
For example, I initially sent out five submissions to agents listed in a writers reference book specifically about travel writing without doing any other research. One agent got back immediately and wanted to look at the whole book, one didn't reply because they were no longer at the address listed in the reference book and the others turned out to be a music book packager, an agent dealing only with theatre scripts and one agent with no travel writers on their books.