A 6****** star math book

I don't rate something 5***** stars easily. I mean what's beyond 5? Today I found a book for which a 5 star rating is simply too low. The book is extremely:
- different
- relevant
- interactive ( even more so than some math 'websites' )
- challenging
- fun.
If there is a book which I ever rated as a 5***** it must be Concrete Mathematics from Graham, Knuth and Patashnik. Polynomials, that's how it's called, is like that but much more coherent. It is a * * * * * * rating, to me anyway. Must have, must read, can't wait, buy now.

Oh, eh, it is out of print, sorry. Not for sale. ;-)

Group Explorer 2.0

"... Group Explorer is mathematical visualization software for the abstract algebra classroom. Users can explore dozens of finite groups (and morphisms among them) visually and interactively. ..."
GE 2.0 developer Nathan Carter.

Where GAP is targeted to professional algebra researchers and Mathematica Abstract Algebra to students who already have completed one or two abstract algebra courses and know the ins- and outs of Mathematica, Group Explorer is meant for beginners. A beautiful tool to demonstrate properties of a limited number of small groups. Version 2.0 is primarily a "look and feel" release. The program is free and downloadable as C++ source or executable.

Lie Algebras

e^ A, e to the power matrix A, e to the power {{2,0},{0,1}}.
Matrices as power coefficients.
Is that cool, or what?