I hesitate to recommend this to you, but I DO recommend it as a good candidate for someone with good general history interest and knowledge, but relatively little on the man himself. Charlemagne: From The Hammer to The Cross by Richard Winston is a good read. I just finished it a few weeks ago.

It was written in 1954, and the author is clearly of the WW2 generation. It was a bit jarring, but effective, to come upon Charlemagne's "final solution to the Saxon question." You don't live through WW2 and then walk on eggshells with your choice of words. The subject himself has a pretty impressive credit and debit balance, and it all gets a good airing. Many points should have gotten more time, but in his defense, this is one book, not 7 books, and the historical written record is pretty thin.

Spoilers - There's lots of killing people and breaking stuff, Irene of Athens is a piece of work, and Abul-Abbas the elephant gets a good long mention.

Rosamond McKitterick's Charlemagne is the most up-to-date scholarly work. For a broader perspective, you might want to check out The Carolingian World (2011), which could be used as a textbook for an advanced history class but which I still find to be a straightforward and enjoyable read.

And if you're willing to make the adjustment, there's nothing like going back to the sources. First and foremost is Einhard's Life of Charlemagne, which can be found in many different editions in print and online. I'd recommend the cheap Penguin edition in Two Lives of Charlemagne, which is a solid translation and includes what is still a quality introduction to this text. (McKitterick also opens up her book on Charlemagne with a good discussion of Einhard, who remains our most informative source on the man himself.) Going a bit deeper, you could also check out Carolingian Chronicles, which brings together some of our other most important sources alongside Einhard. Alternatively, you might look into Dutton's great anthology, Carolingian Civilization, which includes both Einhard and selections from the chronicles.

I glanced through as much of Winston's Charlemagne as I could see on Amazon preview. It looks like a quick and fun read, which might be a good place to start, although I would hesitate to take his word as final on many key issues. We now understand the vitality of Merovingian kings, the relationship between church and state, and even the very notion of political power in the early middle ages completely differently from when Winston was writing in the mid-1950s. If you're looking for a quick sketch of Charlemagne's life, Winston's work seems to have weathered well, but I wouldn't trust it as a map to navigate the contours of Charlemagne's career as we understand it today.

followup question - is there a consensus on whether Charlemagne could read and write?

A brilliant world history teacher in high school, some decades ago, mentioned that he was pretty much illiterate though he also kept a quill and ink and paper near his bed. He was quite good at his job. In a Texas high school, during the Reagan era, he showed us Battleship Potemkin. He has since gotten an PhD, and I'm inclined to take his opinion seriously.

Winston's Hammer to The Cross votes YES he could. Winston also comments at some length that he disagrees with conventional wisdom on this point. Clearly, there were also scribes/secretaries to take a letter as he dictated, but he could at least manage himself. Winston even mentioned his tutor's name.