Famed author Tom Bower -- known for his biographies about the Royals, most of which are not sanctioned by the subjects -- has a forthcoming project hitting shelves called "Revenge: Meghan, Harry and the war between the Windsors" ... and, boy, does he get into it.

Per Bower ... there were subtle shots taken between both parties behind closed doors and several dust-ups these past handful of years ... which seems to have stemmed from Meghan's Vanity Fair cover in 2017, the first time she publicly acknowledged the relationship.

It was a relatively small part of the article, with Meghan saying ... "We're a couple, we're in love," and going on to say they'd eventually come forward and tell their story in full. But, of course, VF's headline was all about their relationship -- "She's Just Wild About Harry!"

Apparently, according to Bower, the Royals were pissed -- because they'd specifically instructed Meghan NOT to talk about Harry. At the time, they'd been seen together publicly, but neither had spoken to the press. Meghan, in turn, reportedly got up in arms with her PR agency ... telling them to demand VF pull the quote and change their angle. Vanity refused.

That doesn't seem to have left a great impression with Buckingham Palace -- as we know, tensions only grew as time went on ... reaching a boiling point with the Oprah interview.

The aftermath of that sit-down proved to be devastating, so says Bower -- evidenced in Queen Elizabeth II allegedly saying something to the effect of ... "Thank goodness Meghan is not coming," when Prince Philip died a month after the interview aired. MM was pregnant again at the time ... and only Harry went to the funeral, but even he was said to have been antsy while there.

Of course, when Lilibet was actually born ... there were reports that Meghan and Harry sprung the name (Liz's actual nickname) on the Queen -- which Bower also apparently confirms in his book.

Finally, there's the drama that was quite obvious at the Queen's Platinum Jubilee earlier this year -- when Harry and Meghan were more than arm's length from the rest of the family pretty much throughout their entire time there ... especially for the coveted balcony seating.

As it turns out, according to Bower, MM and PH were dead set on getting a spot next to the Queen -- this, Bower claims, in return for what they thought they were owed for bringing the kids out there in the first place -- but Prince Charles shut it down ... and the Queen quietly cosigned by not granting their wishes. As you know, Harry and Meghan left soon afterward.

There's also the issue of Prince Harry's interview with Hoda Kotb, where he kinda dissed his brother and dad -- while making it seem like he and nana were perhaps even closer than she was with any of her other children/grandkids. The Royals resented this too, Bower claims.