I have done peer reviewing of philosophy/cognitive science books (monographs) for publishers, sometimes on a detailed book proposal/outline, and, in one case, on a complete manuscript (which I strongly recommended should be rejected, incidentally). Unlike reviewing for journals (which I have also done) I was paid a small fee.
I think when we are talking about edited volumes of essays, rather than monographs, the publishers rely on the volume’s editors to ensure academic quality. The essays will generally be by authors specifically invited by the editors. It is not as rigorous a process as the peer review used by journals, but there is still quality control by the editors (who will themselves have to have convinced the publishers that they have a sufficiently good reputation to be able to do the job and produce a salable book). Probably book editors do sometimes send out contributions for external peer review also.
I expect it works the same way, more or less, for academic history, although the fact that many historical monographs are written for a relatively wide audience, not just academics, may make a difference, such that more ordinary considerations of marketability to a popular audience come into play. I would not be at all surprised if quite a bit of academically unsound history gets published, because the publisher think it will sell, perhaps because its takes an extreme position, or flatters certain political prejudices. This is not going to happen so much in most other academic fields, where the demarcation between the professional and the popular literature is usually much clearer.