Magnus and Alec decamp to the glittering City of Light for their first-ever vacation together.

They gaze at Paris from atop the Eiffel Tower observation deck, glide over the Seine in a hot air balloon — and watch a swarm of demons soar over the rooftops, which means it’s time for Alec to get out his bow and arrows.

Such is life for a couple in Cassandra Clare’s universe. Clare unveiled the “Mortal Instruments” world of Shadowhunters, part-angel warriors who police the demons on Earth, in 2007 with “City of Bones.” Since then, she’s written five more "Mortal Instruments" books, a three-volume prequel series, two sequel trilogies, and several stand-alones – plus co-authored an entire other series. There was a “Mortal Instruments” film in 2016 starring Lily Collins, and a “Shadowhunters” television series on Freeform.

Now comes “The Red Scrolls of Magic” (Margaret K. McElderry Books/Simon & Schuster, $24.99), the first book in the “Eldest Curses” series featuring reader-fave couple Magnus and Alec. Clare co-authored “Scrolls” with Wesley Chu, and both will be in Austin on Tuesday, the day the book publishes.

“In 2012 I published ‘City of Fallen Angels,’ and … Magnus and Alec are significant supporting characters. During that book, they disappear on vacation, and they come back and won’t talk about it, but it’s obvious that something important happened,” Clare explained recently in a phone interview. “I always had the thought that I would go back and write the story of what happened to them during that time period.”

Part of the challenge was finding time to write “Scrolls” between other commitments. And just as crucial, she says, was having a story centered on a gay couple be promoted and treated like any other book.

“I was often told that school libraries wouldn’t carry these books, or that I wasn’t welcome to speak at a school because I feature these characters,” she says.

With the increased push for inclusion of all types in young adult literature, the market was ripe for stories of all kinds featuring gay characters, from coming-out stories to tales like Clare and Chu’s, which are adventures in which the main characters happen to be gay.

Authentic inclusion also prompted Clare’s push to have a co-author like Chu, a celebrated science-fiction author who shares Magnus’ Asian heritage: “We’re really digging into Magnus’ history, and Wes is a man of color who could speak to Magnus’ background. … It’s a significant responsibility,” Clare says. “You can’t walk away from the world you’ve created; you have to make it more inclusive. You’re going to have to write the experience that’s not your experience. The only other choice is to leave them out entirely, and that would be wrong.” (Ages 14 and older)

Also coming to Austin in April

The porcine wonder Mercy Watson has starred in six New York Times-bestselling chapter books from author Kate DiCamillo and illustrator Chris van Dusen. Now the pair have teamed on a prequel, “A Piglet Named Mercy” (Candlewick Press, $18.99), the picture-book story of how Mercy came to live with the Watson family.

Mr. and Mrs. Watson live an entirely unexceptional life on Deckawoo Drive. Mr. Watson is satisfied with their predictable existence. Mrs. Watson, not so much: “It’s just that sometimes, I wish something different would happen.”

Enter a tiny piglet, who falls off the back of the farm truck and lands on the Watsons’ front porch next to the newspaper. A busybody neighbor (and her entranced younger sister) round out this origin story.

DiCamillo and Dusen will be at BookPeople at 6:30 p.m. April 16.

(Ages 2-5, although older fans of the chapter-book series also will be interested.)

Adam Rubin and Daniel Salmieri paired to create the “Dragons Love Tacos” empire of picture books. Now they’re (figuratively) holding up their hands and inviting young readers to do the same with “High Five” (Dial/Penguin, $19.99), a picture-book tutorial on extreme high-five.

Can your young reader do the “supermarket sweep”? The “king of the jungle”? The “windmill”? Whimsical rhymes from Rubin, who will be in Austin later this month, accent Salmieri’s simple and colorful pencil drawings, all printed on heavier-than-usual paper to encourage an interactive experience.

Slap palms with Rubin at BookPeople at 4 p.m. April 29, which also happens to be the first day of the 100th annual Children’s Book Week — the longest-running national literacy campaign in the United States. (Ages 3-5)

Save the date for Texas Teen Book Festival

Calendar alert! This year’s Texas Teen Book Festival will be Oct. 12 at Southwestern University in Georgetown. Now in its 11th year, the event features a full lineup of young adult authors and is a collaboration between the Texas Book Festival and BookPeople.

(Update: The name of character Alec has been corrected.)