This script recursively deletes e-book library directories that have no books in them. It does this by checking for empty directories or directories with only OPF files in them. It then runs again to check for empty directories (to include the directories it may have just emptied).

deleteEmptyOrOPFOnlyDirs.ps1

$ebookLibraryPath = '.\OneDrive\Documents\Ebook Library\' 
Function Get-Empties {
	PARAM($targetPath)
	Return Get-ChildItem -Path $targetPath -Recurse | Where { $_.PsIsContainer -eq $true } | Where { $_.GetDirectories().Count -eq 0 -and ($_.GetFiles().Count -eq 0) -or ($_.GetFiles().Count -eq 1 -and $_.GetFiles().Name -match "(?i)(.*?)\.opf$")}
}
$failed = @{}
$toEmpty = Get-Empties $ebookLibraryPath

Function Write-Log {
   Param ( [parameter(Mandatory=$true,
    ValueFromPipeline=$true)]
    [string]$logMessage,
    [PSDefaultValue(Help = "Will create a log file in the same directory as the script is run")]
    [string]$Logfile = ".\$(gc env:computername)-delete-dirs.log")

   Add-Content $Logfile -value $($(Get-Date).ToString() + ": " + $logMessage)
}

while($toEmpty.Count -gt 0) {
	try {
		foreach($dir in $toEmpty){Write-Log "Deleting $($dir.FullName) which has LastWriteTime $($dir.LastWriteTime.ToString())"; Remove-Item -LiteralPath $dir.PSPath -Force -Recurse -Confirm}
	}
	catch {
		$ErrorMessage = $_.Exception.Message
		$FailedItem = $_.Exception.ItemName
		Write-Error -Exception $_.Exception
		$failed.Add($failed.Count+1,$dir)
	}
	$toEmpty = Get-Empties $ebookLibraryPath
}