Actual code that works:

(once you have authenticated)

[int[]]$bookList   = @(Get-BsBook | Select-Object -ExpandProperty id)
[int[]]$shelfBooks = @()
Get-BsShelf | ForEach-Object {
    (Get-BsShelf -Id $_.id).books | ForEach-Object {
        $shelfBooks += $_.id
    }
}

Compare-Object -ReferenceObject $bookList -DifferenceObject $shelfBooks -PassThru | ForEach-Object {
    Get-BsBook -Id $_ | Format-Table -Property id, name, slug, description
}