If Mango seems a bit much, one might try Goldsworthy's Caesar The Life of a Colossus, but Goldsworthy, while totally thorough and taking one through Caesar's entire career, including the entire Gallic War, is a complicated read and as we used to say as kids, "deep," so he's not a leisurely read. But thorough and scholarly which is what is needed.
For the real skinny and in a readable short paperback format, one might try the small new book Julius Caesar by T.P. Wiseman, in a series called Pocket Giants. T.P. Wiseman is THE authority for Caesar today in the world, and his little book here is chatty but full of the real facts, and the knowledge of things (like Antony's famous speech which had elements Shakespeare forgot to mention and nobody will ever forget). His books are normally spectacularly expensive for a little book, so this one, a nice inexpensive paperback, is a real treasure.
His Remembering the Roman People, also a fine expensive small hardback with special attention to the chapters After the Ides of March and the Ethics of Murder is especially wonderful, too, if wallet busting. He's supposedly the model for Dumbledore in the Harry Potter series because J.K. Rowling, the author, had him for classics at Exeter. He says he can't see it. hahaha
Wiseman, normally, does a lot of footnotes, but not in the new paperback Julius Caesar. Normally if he says something he footnotes it and shows you exactly where it came from, translating if he has to, and all that on the same page: old tyme scholar. I love him. But he's not doing that in the little Julius Caesar paperback (or trying not to) and so it reads like a novel, while breathtakingly on point.
Ordinarily his footnotes are more interesting than most people's texts, if you want one of his more expensive books.
I'd always read him first, last, and always, but Goldsworthy has all the details and explanations about the Gallic War, too, and I think, in that order, they are the best out there today.
Reviews for Remembering the Roman People and I could put like reviews here for Goldsworthy, too:
"Wiseman's very selective use of secondary material stands out. This is one of the most learned of classical scholars, who knows the fields, both ancient and modern, inside out, and who argues meticulously against individual positions."
--Christina Kraus, Bryn Mawr Classical Review
"This book is ground-breaking for its simple suggestion that the ideology of Roman popular politics is not entirely lost to us, and for its virtuoso demonstration that, fragmentary, inadequate and intensively studied as our sources for the period are, they may still have more to tell us." And for the previous edition of the same book: "The importance of his work lies not only in what he argues but in how - and in the vision of the Roman past he invites us, with such enthusiasm and elegance, to share."
--Mary Beard, Times Literary Supplement