Upload monograph to preprint repository before acceptance for publication? Can one upload a book-length monograph to a preprint repository (e.g., ResearchGate) while awaiting for its acceptance for publication?

(Book preprints do exist on arXiv.org, e.g.: Bacciagaluppi &amp; Valentini's Quantum Theory at the Crossroads: 0609184v2.)
You have to check with the publisher, or read their posted terms and conditions.  For some publishers it might be fine.  For others, they might insist on having the exclusive rights to distribute the work, and if you've already uploaded it to a preprint repository, that would make it impossible for the publisher to be the exclusive distributor.  In that case they might refuse to publish your manuscript.