I have been using Liberty Math K with my 4-year-old daughter, and we both love it! There are no frills, which is perfect for an easily distracted little girl, and yet the material is thorough and progressive (i.e. it begins easily and slowly builds upon itself as the book continues). I love the slower pace of the book, as it does not throw too much at her at once, and she can take her time and really learn the material. As well as working with the numbers, there is a lot of practice writing the numbers, and her visual recognition of numbers from 1 to 100 has greatly improved. Overall, a great beginning math book!
I used this for the first time this year and we like it. My son is a little more advanced so he has flown right through it without any trouble. The directions are easy to understand and there is lot's of practice for writing numbers!