by all means encourage her with books etc but please dont worry just yet!
i started worrying whn willow was 18 months, and again at 2 years because all the other kids i knew were fluent and practically reciting Shakesphere.
I had 2 assessments done with speech and language therapists, one was a trial thing for half hour so was free, the other was a proper full on assessment by my friends sister so luckily that was free too, if you start on that route be prepared to pay over £100 for an initial assesment and then £50 an hour there after! shocking!!
Each assesment they said she was fine for her age, i still worried! but now her words seem to be coming thick and fast and she is copying more.
She was never behind in the first place but comparing her to the other kids who talked more made me paranoid.
At 18 months she really is too early for even an assesment to be worth while.
Her words will come, what you have to work on is trying not to compare to others! which is really really hard! lol especially in the world where as soon as your baby is born, EVERYTHING becomes a competition between babies/toddlers! drives me mad!!
Does your daughter go to nursery?
Ive found that all those kids who talked early and were taking their GCSE's at the age of 2 were all in childcare early on where they were exposed to more language from different people hence their fast learning.
Willow only goes twice a week and started when she was 15 months so not as early as others. Thats what i kind of think anyway!

x