A very well-written, highly mathematical novel for 5th — 6th graders. Three children — Shawna, Adin and Tavia — are math aficionados and are trying to crack the Beale Ciphers, a set of 3 documents from the early 1800s which purportedly encode information about a treasure. They don't succeed, of course, but along the way, they run into many interesting mathematical topics like factorials, Fibonacci numbers, triangular numbers, substitution ciphers, Pythagorean numbers, etc, all of which are woven into the novel's narrative quite well.
A particular example from the book: As the 3 children were heading home once, they saw a very large crane which was being set up to be used for bungee jumping. The crane had to be 120 feet high for it to be safe for such jumps but the operator was not sure nor did he care of it was 120 ft high — he simply assumed it was so because this new crane was similar to the old. The children, worried that ths would lead to an accident, decide they must verify the crane's height. With an inspired thought, they decide to use their shadows and the geometry of similar triangles to carry out the measurement. Accurately measuring Tavia's shadow to be 9 strides / feet long, the crane's shadow to be 198 strides and Tavia's height of 5 feet, they deduce the crane to be 198/9*5 = 110 feet high…