Sorry this took me so long, and I forget who it was I promised info to, but my SALT friend finally got back to me!
She spoke to people she works with who have done baby signing with their children and they have recommended the Makaton system as it's linked to BSL so will crossover with any signing done in nurseries and schools.
The Makaton website (www.makaton.org) has lots of information on it but the main books they do are:
-Signing for Baby's Pocket Book
-My First Makaton Symbols and Signs books 1, 2 and 3
She also had this to say about signing aiding development:
"Just a little extra she told me was that the Royal College of Speech & Language Therapists have done some research into the effectiveness of baby signing and basically if your child is typically developing the signing won't make your child learn language any faster or better but it does give your child more ways to understand and express themselves which is definitely a good thing. However if your child does have and other difficulties or has delayed language development then it will help greatly."
She did also say that while Makaton is a good idea from her standpoint it doesn't really matter which system you go with as long as you're consistent with it.
HTH!