I think what you're doing is great, and I'm glad Alex loves letters. I just wanted to add the data point, especially for your readers who don't know that much about child development, that my normal, intelligent, verbal 3.5 year old doesn't recognize letters reliably, can't tell you what sounds start most words, and in general is not showing a lot of signs of pre-reading. And I think that's normal, and fine, and we do all the same normal book-loving household things that you're doing (reading all the time, letting her fill in words, talking about letters, etc.) My kid is a whiz aurally, but doesn't seem to make the word connections with the little bits on the page.

In other words, Alex is exceptional in this area (as in so many, of course!), and that's wonderful, but others shouldn't be worried (or jealous) if their kid isn't. Plus, you'll probably be needing to lock posts against her reading them soon, and I don't expect to have to do that until 5 or 6, anyway!