Bisclavret by KL Noone (M/M fantasy romance): This is such a sweet, lovely retelling of Marie de France's Bisclavret story. In it, a man turns into a wolf a few days a month and when his wife finds out, she steals his clothes, condemning him to remain a wolf. He's found by a king (in this version, demisexual) and is saved. The slow burn romance between the king and Lord Bisclavret is wonderful.
The Painted Crown by Megan Derr (M/M fantasy romance): I always enjoy Megan Derr's fantasy romances, and this one is no different. It's the second book in the Unbreakable Soldiers series, but you might be able to read it as a standalone (though the first story was great too). I'm highlighting this one because both main characters are demisexual, and their growing relationship—getting to know each other, becoming friends, bonding—is well done.
In the Ravenous Dark by AM Strickland (upper YA/NA fantasy romance): This is the only book I'm listing that has an asexual supporting character instead of a main character, but I did want to list it because the supporting character (who is asexual and nonbinary) is a major character and absolutely fantastic. This fantasy has combined a fascinating magic system of blood and death magic, an almost entirely queer cast and wonderful found family, ghostly guardians, a creepy underworld, and intriguing world building into something unique and engrossing.
The Grimrose Girls by Laura Pohl (YA fantasy with romance): This YA dark academia book gives us a blend of mystery/thriller and fantasy with fairy tale retellings. Before school begins at the elite Grimrose Academy, a girl dies. Everyone believes it's suicide, except her closest friends who set out to learn what really happened and uncover something darker than they imagined. There is so much diversity of representation, and Yuki, one of the main characters, is asexual.
The Not Dead Yet series by Jenn Burke (M/M paranormal romance): I talked about this series last week in my vampire book rec post, but I wanted to mention it here as well because one of the main characters, Wes, is demisexual. There are also fun paranormal mysteries, wonderful found family, and a fun second chance romance.
A Gentleman's Position by KJ Charles (M/M historical romance): A Gentleman's Position is the third book in the Society of Gentlemen regency trilogy, and best enjoyed after you've read the other two. I'm highlighting this one because it's when Richard Vane, the demisexual character, becomes the main character, though he's played a significant role in the two prior books. Richard has long been in love with his valet (and person who fixes all problems for him), but since David works for him, Richard insists on ignoring it. Which also means he's ignoring what David wants—except David refuses to be ignored. There's also a whole tangle of scandal for David to fix and a perfect wrap up to the whole series.
The Rat-Catcher's Daughter by KJ Charles (trans F/M historical romance): Yes, I'm listing another KJ Charles, this one a novelette prequel to the Lilywhite Boys series. Both main characters are asexual in this little gem. Miss Christiana is a music hall singer, and Stan is a massive fan and a fence for the notorious Lilywhite Boys. And they are absolutely wonderful together. It's short and sweet, and a perfect introduction to the series.
Far From Home by Lorelie Brown (F/F contemporary romance): This is a contemporary marriage of convenience story—Pari needs a green card and Rachel needs some relief from drowning in student loan debt—with a demisexual character. Throughout the story, we see them getting to know each other and of course falling for each other (because a fake marriage never stays that way in romance, right?). It's the first book in a series, but each stands alone.
I don't usually mention my own books in my book rec posts, but I have written four books with ace spec main characters and I wanted to list them for you in case you were looking for some more fantasy romance with ace spec characters.
To Love the Dragon King: King Lysander has learned of a plot against him and the kingdom and set out to discover all the traitors. He encounters Sascha, who has unknowingly been used as a pawn by his family in that very same plot. There is a lot to be done to build trust between them and keep them all safe. Plus this one has dragon shifters. To Love the Dragon King is the first book in the Dragons of Ivria series, but doesn't end on a cliffhanger. (Sascha is gray ace.)
The Merchant's Love: I constantly call this one a warm hug in book form. It is the coziest of cozy fantasy romances in which a bookish royal and a merchant who is besotted with him from first sight become friends and fall in love over books and baked goods. And there is just a bit of magic gone wrong. It's the sixth book in the Chronicles of Tournai series but can be read as a standalone. (Faelen is demisexual.)
A Dance of Water and Air and A Harmony of Fire and Earth: These two books make up the Elemental Magicae series and should be read in order. In A Dance of Water and Air, Prince Edmund must marry the queen of a neighboring country in an arranged marriage, but when he arrives, he becomes close to her brother, Arden, and falls in love with him instead. Then Edmund is accused of trying to kill the queen and he and Arden flee, so they can try to unravel the mess affecting both their countries. (Edmund is demisexual.)
Do you have any favorite books with asexual characters?