I would try to fish as much out of her as you could to get an idea of why the sudden interest in these things, but at the same time, don't fob her off with vague answers. I don't mean that you have to tell her in great detail, but they know and understand more than what you think - don't assume that it would be above their heads to talk about love etc. The way I see it, it is best to come from us as the parents than some older kid with a dirty magazine kwim?

Some books I have found to be great are Dr Miriam Stoppard's "Questions Children Ask" which covers almost all the hairy questions and the answers are graded to suit different age groups. Another great book is "How did I begin?" which covers the baby question really well. You know your daughter best and know what she would be able to handle.