Hello Math Readers easy readers at all levels. I have not found a site that lists them all. You can search on Amazon for a listing of what is carried. Each book clearly indicates what concept is covered. Most books are for early elementary (K-4th).

Fun With Math with Games and Puzzles by Lakshmi Hewavisenti- I have found 2 of these, there is an entire series, Amazon has 3:   Problem Solving       Shapes and Solids       Counting

MathStart Readers   See Complete List of Readers by Concept

Time Life "I Love Math" - Out of print, many are still available used, my kids have loved this series

Alice in Numberland (Fantasy Math) original stories, poems, riddles, games, and hands-on activities reflecting Alice's adventures in Numberland, where she finds mathematical challenges throughout the magical landscape
The Case of the Missing Zebra Stripes Math thru a trip to the zoo
City Math: Look Both Ways       1992, Math in Life
From Head to Toe, Body Math
Look Both Ways: City Math
Nature Math: Right In Your Backyard  z 1992  Math in nature. Beautiful pics, well done book
Pizza Math, How Do Octopi Eat Pizza Pie? z 1992 - Fractions, my kids love this one
The House that Math Built
The Mystery of the Sunken Treasure: Sea Math
Play Ball - Sports Math
Pterodactyl Tunnel: Amusement Park Math
See You Later, Escalator!: Mall Math Stories, poems, riddles, games, and hands-on activities introduce early math skills, focusing on the mathematics one might encounter on a trip to the shopping mall.
The Search for the Mystery Planet Math and astronomy topics
Zoo Math: Case of Missing Zebra Stripes,  1992  Math in nature, my kids really like this

Young Math Series - Out of print, hard to find, nice when you do . .
3D 2D 1D   Adler, David Geometry, measurement, volume/area. Very readable
Base Five    Adler, David/Ross, Larry   1975   Number base systems
Estimation   Linn, Charles/Madden, Don  1970 Activities, simple intro
Circles and Curves  Razzell, Arthur/Watts, KGO/Raskin, Ellen   Geometry
Counting Systems   Luce, Marnie  1969   History & description of number systems
The Ellipse     Charosh, Mannis/Kessler, Leonard  1971 Geometry. Lots of activities
How Did Numbers Begin? Sitomer, Mindel&Harry/ History of numbers Young Math book
Number Families    Srivastava, Jane Jonas/Ehlert, Lois  1979 Number series
Number Ideas Through Pictures   Charosh, Mannis  1974  Odd/even,square/triangular numbers
Odds & Chances for Kids   Riedel, Manfred 1979 Probability, history, stories. Much is in storytelling form
Statistics by Srivastava, Jane Jonas 1973   Excellent intro to statistics for younger
Straight Lines, Parallel Lines, Perpendicular Lines  Charosh, Mannis  1970 Geometry activities
This is 4: The Idea of a Number, Razzell & Watts, 1964, relationships with “4”
Yes No Stop Go, Patterns in Mathematical Logic  z Gersting, Judith & Kuczkowski  1977  Patterns and logic.
Shadow Geometry by Daphne Harwood Trivett
Roman Numerals by David A. Adler
Lines, Segments, Polygons by Mindel and Harry Sitomer
Odds and Evens by Thomas C. O'Brien

I have not found or obtained these, but they are part of the series:
666 Jellybeans! All That? An Introduction to Algebra by Malcolm E. Weiss
Angles are Easy as Pie by Robert Froman
Area by Jane Jonas Srivastava
Averages by Jane Jonas Srivastava
Bigger and Smaller by Robert Froman
Binary Numbers by Clyde Watson
Building Tables on Tables: A Book about Multiplication by John V. Trivett
Circles by Mindel and Harry Sitomer
Computers by Jane Jonas Srivastava
The Ellipse by Mannis Charosh
Estimation by Charles F. Linn
Exploring Triangles: Paper-Folding Geometry by Jo Phillips
Fractions are Parts of Things by J. Richard Dennis
A Game of Functions by Robert Froman
Graph Games by Frédérique and Papy
The Greatest Guessing Game: A Book about Dividing by Robert Froman
How Little and How Much: A Book about Scales by Franklyn M. Branley
Less than Nothing is Really Something by Robert Froman
Long, Short, High, Low, Thin, Wide by James T. Fey
Maps, Tracks, and the Bridges of Konigsberg: A Book about Networks by Michael Holt
Mathematical Games for One or Two by Mannis Charosh
Measure with Metric by Franklyn M. Branley
Probability by Charles F. Linn
Right Angles: Paper-Folding Geometry by Jo Phillips
Rubber Bands, Baseballs and Doughnuts: A Book about Topology by Robert Froman
Solomon Grundy, Born on Oneday: A Finite Arithmetic Puzzle by Malcolm E. Weiss
Spirals by Mindel and Harry Sitomer
Statistics by Jane Jonas Srivastava
Venn Diagrams by Robert Froman
Weighing & Balancing by Jane Jonas Srivastava
What is Symmetry? by Mindel and Harry Sitomer
Zero is not Nothing by Mindel and Harry Sitomer