I'm not aware of any smaller books that are actually acting as 'the book'. These days there are the giant established books, and then there are a few odds providers that run the back end of most crypto betting sites.
I would expect the established books to take the hit and pay Trump bets. If they didn't, it would do serious damage to their reputation. (Paddy power actually paid out Clinton bets a week or two before the 2016 election as a publicity stunt. In the end they probably still made out ahead after the MSM ran with it and gave them tons of exposure)
For the rest I would expect the odds providers to take the hit, and if they ran off then you'd see some smaller books that don't have the cash or rep to protect fold up.
I know Stake came out and said they were paying Biden bets out of pocket because their odds provider was planning on waiting till Jan and they didn't want all the money tied up for that long. They were open about being on the hook if Trump does find a way to win.
@OgNasty a few days back this would have been a genuine concern for all the parties i.e. gamblers and sportsbook, but now Biden victory is official, and nothing can change it.
Furthermore even Republicans are accepting him as the president, which ends any speculation about Trump being able to reverse the election results, hence we don’t need to worry about this scenario.
Lastly even if we consider a hypothetical scenario any legit sportsbook would be foolish not to pay those people who wagered on Trump, because they would lose a lot of money in the long run as people would stop playing on their sites.
Sources:
https://apnews.com/article/electoral-college-confirm-joe-biden-win-2d4fd7368d8fd6cb47ff0b2cc206271a
https://www.washingtonpost.com/nation/2020/12/16/trump-mcconnell-election-biden-president/
https://www.nytimes.com/2020/12/15/technology/can-trump-still-win.html
Technically Congress still has the chance to object. And a few will, and then they'll have to debate, and then as long as a Majority of both the house and senate don't agree with the objections then he will have the votes.
And then, technically, he won't be president until sworn in.
It's kind of like when a golfer wins a tournament. He's still gotta sign the score card, and then he's still gotta do the ceremony, but the books want the money in the gamblers hands asap so they can gamble with it, so they aren't going to wait unless there's a good reason to.
Did people bet on who would be president in February 2021 or did they bet on who would win the election ? Because the election is over, and Biden has won that. The only thing that can happen now is some kind of coup, either by judges, the GOP, or by force. None of which are very likely.