In a yandere phobos x will fanfic, one aspect to consider is the power dynamic. If Phobos has more power, it can intensify his yandere tendencies. For example, he could use his influence to isolate Will. The fic might also focus on Will's reaction to Phobos' yandere behavior. Will could either be scared or, in an interesting twist, gradually come to understand and even accept Phobos in some way.
Look for fanfic communities dedicated to the source material that Phobos and Will are from. There, you can ask for recommendations of 'yandere phobos x will fanfic'. Members are usually quite helpful in sharing their favorite stories in this genre.
I'm not aware of the author of this 'Phobos King of Beasts' novel. It could be an indie author or someone from a relatively unknown publisher. Without more information, it's impossible to determine the author.
One possible plot could be that they kidnap a character to gain some sort of power. Maybe they think the kidnapped character has a special ability or knowledge that they need. For example, if it's in a superhero - themed fanfiction, the kidnapped character might be able to control an element, and Will and Phobos want to use that power for their own evil plans.
It could be a young and innocent bystander. Maybe someone who was just in the wrong place at the wrong time. For instance, a small child who happened to be near where Will and Phobos were planning their kidnapping and was grabbed instead of their intended target.
I'm not sure specifically as there could be many novels with 'the librarian' in the title. It might be about a librarian's daily life, adventures in a library, or perhaps a mystery involving a librarian.