I agree with the others! Especially the baby sign and "The science of Parenting" - I think Kelly mentioned it is now called What Every Parent Needs to Know. (Margot Sutherland) I picked up a copy from the library and it really spells out clearly how a child's mind works.

With baby sign, we started at 12 months - same as you, meant to start earlier but never got around to it. By 13months DD was signing her first word and absolutely took off from there. Her verbal development was astronomical after that too as we always used words with the signs. She really loved it and on more than one occasion it saved a tantrum as she was able to sign what she wanted when she couldn't say it.

Instead of investing in Baby sign books, just check out the Auslan website. They have short videos of someone signing every word you can think of. I started with two simple signs we use all the time, Milk and book. She picked up milk first and then realised what was going on and looked for my signs for everything. I was constantly checking Auslan webpage!!