All About Reading. It's not cheap but it's fun, a great foundation, and super easy to use. An Ordinary Parent's Guide to Teaching Reading is less "fun" but incredibly effective and inexpensive.
With a kid that's picking it up fast anyway (went through that with LegoMan), I'd get OPGTR to cover any gaps and then once he/she is reading well, move on to All About Spelling to ensure that all phonics and spelling rules are covered.