Hard Math for Elementary School by Glenn Ellison: (ISBN 9781489507174) looks like a book I could have used with my son about 8 years ago (too bad it was just published a couple of months ago).
The premise is simple: it is a math enrichment textbook, intended to take the topics of elementary school math deeper for gifted students.
The presentation is good, but the students will have to be reading at the 4th grade level as well as doing math at that level to get much out of the book. This is not a flashy book with lots of illustrations and stories—it is just cool math, presented as cool math.
Disclaimer: I don’t have a copy of the book, and I haven’t read much of it. I used Amazon’s “Look Inside” feature to look at the table of contents and a couple of pages, and saw such gems as calculation tricks for computing some squares (like 552=3025) quickly. (The trick is based on , but the author wisely avoids algebraic notation.)
Reviews from people who have looked at it in more detail can be found at http://albanyareamathcircle.blogspot.com/2013/05/recommended-summer-reading-for-young.html and http://blog.supplysideliberal.com/post/53817714552/glenn-ellisons-new-book-hard-math-for-elementary
Glenn also has a book for middle school students: Hard Math for Middle School