After thinking a bit I thing it would be better to use the entity service api, so that you can have more control over your request if you need it (sort, start, limit…).
Something like this:
const result = await strapi.entityService.findOne(‘api::bookshelf.bookshelf’, {
where: {user: {id: id}},
})
Related docs: Entity Service API | Strapi Documentation
Glad I could help 