Beevor tends to simply just make statements and present them completely as fact. The book also tends to be fairly opinionated such as making character judgements about key actors during the war. I also have been reading Berlin: The Downfall 1945 also by Beevor which does include a fairly extensive list of references and sources and so was wondering why sources were not included.
Are you sure there aren't any in your copy? I ask, because I opened up my copy, and pages 788 through 833 are devoted to endnotes, listing a wide variety of sources, examples:
p. 723 'The Germans just don't seem to understand':GBP, 2/4/45 'It is the fear of Russia': Blumenson (ed.), The Patton Papers, vol. ii, p.22.11.44, p.580
p.726 'not the logical':Eisenhower, Crusade in Europe, p.433
He doesn't include the superscript note in the text itself, but this isn't that uncommon with mass market books written for a general audience, as opposed to an academic one. So anyways, point is, there are notes and sources at the end of the book.
Edit: I just checked a few other books of his I have, and this is the same style citation he used in D-Day, Berlin, Crete, and Stalingrad.
It's a mass - market book, aimed at popular audiences rather than historians, or even students of history. Publishers of these kinds of books often don't want to print footnotes, endnotes, or even bibliographies because it cuts into profits and (they assume) scares away casual readers. It's possible Beevor wrote the book with notes and the Publisher made him take them out, or its possible he knew in advance that they'd never be printed and didn't bother.
Unfortunately it's a pretty common practice in the publishing world - much to most historians' chagrin. I've met some very senior, respected scholars who have published books like this (usually late in their career) and they all wanted to include notes in their books. Leaving them out is a kind of deal with the devil that they make to reach a broader, more popular audience. A good, responsible historian will have kept track of their references and sources somewhere, though, in case their narrative or their research is ever challenged.
I just finished about a week ago, and noticed the same thing. But, isn't that acceptable for lay-accessible history books? I would expect a more conversational style, with the occasional subjective judgment or rhetorical exaggeration to make a point in a book written for the general public.