So sad for those of you caught in this. It's awful. Honestly, between the fires, the floods, and the pandemic this year...well...
Christina, that's horrible about your husband's books. I'm so sorry.
You may know this, but just in case, a trick that might help, if you can manage and you still have power -- buy or rent a massive dehumidifier (if you can afford, of course) and/or get some fans running. Of course you need to ensure electrical use in the area is safe (and it might not be, in the basement itself -- you might have to cut the fuse and use extensions from elsewhere in the house). If there are appliances in the basement, best to shut off the power to that area if at all possible.
I've had basement floods and lost a lot of books, but some were salvageable, so it just depends. They will definitely need sun and airing.