Book 9 for my 2025 goal of 30 books for the year! Opted to switch to some history of science and mathematics to mix things up…

Some books in my collection were things that caught my attention at the time I bought them, but that I somehow never got around to reading right away and they disappeared into the pile of “to read” for a long time — years, even. Such is the case with the intriguing 2011 book The Man of Numbers, by Keith Devlin.

The book is about the accomplishments of the man often referred to today as “Fibonacci,” although that was not his name and he never used it himself — his name was Leonardo Pisano, “Leonardo of Pisa.” As the book explains, it was a nickname given to him by a 19th century historian based on Leonardo also referring to himself as “filius Bonacci,” of “son of Bonacci.” Leonardo is today credited with being the man who brought the Hindu-Arabic systems of numbers and algebra — the system that formed the foundation of most of modern mathematics — to Europe. He wrote a beautiful and thorough book titled Liber abbaci (the book of calculation), and the success of this book led to many imitators and derivatives that spread like wildfire through the merchant classes of the continent.

What is striking about The Man of Numbers — and this is readily admitted by the author at the start of the book — is that we know very little about Leonardo of Pisa’s personal life, aside from a few broad biographical details. The book instead describes the era that Leonardo lived in to give us a sense of what his life and efforts were like, and describes in detail the mathematics that he worked out in his famous book. The era of Leonardo — the 13th century CE — is itself unevenly chronicled, and this presents unique challenges to an author writing a popular account. To me, the book is the equivalent of painting a portrait of a person where the actual image of the person is itself left as an empty white space. We must rely on the background of the painting to deduce what sort of person would most naturally fit in that space.

Though it is a popular account of the life of Leonardo, this book is probably not for everyone. With a lack of a detailed biographical narrative, Keith Devlin talks a lot about the scholarship and research related to Liber abbaci and its lasting influence and a lot about the mathematical problems contained in the book and their significance. If you’re the sort of person who hates mathematical word problems, you might suffer a bit through these descriptions. For me, as both a history and a mathematics nerd, I got a kick out of all of it — though I also skimmed a number of the particularly lengthy math problems.

The math is quite key to the story, however: seeing how mathematics was done before and after Leonardo introduced the Hindu-Arabic system to Europe, and the form the new mathematics took, is essential to understanding the revolution that occurred. I think most people today would consider arithmetical problems to be somewhat dull, but in the time of Leonardo the ability to solve such problems efficiently was a revelation.

The book is short, only 158 pages of main text. Again, this is a reflection on the lack of material about Leonardo of Pisa but it also means that the book doesn’t wear out its welcome. We get the essential information about Leonardo, his work, and its influence on modern mathematics and the whole thing can be read in a couple of days of careful perusal.

Even with relatively few details about Leonardo himself, his story is remarkable and quite awe-inspiring. It has been broadly confirmed that his writings — the writings of a single man — forever transformed mathematics and commerce in Europe. It is also astonishing that his work was very nearly forgotten completely, drowned out by a flood of derivative works. Only a brief mention of his name in a more modern text (by which I mean tens or hundreds of years after Liber abbaci) led a historian to track down one of the few precious original copies of Leonardo’s book.

So, in short: if you love mathematics and history, The Man of Numbers is a fascinating look at one of the most influential practitioners of mathematics of all time. The book is somewhat limited by the paucity of information about the man himself, but I nevertheless found it enjoyable and rewarding.