diff --git a/lib/EbookDownload.php b/lib/EbookDownload.php
index 18ca2fdf..250a94f5 100644
--- a/lib/EbookDownload.php
+++ b/lib/EbookDownload.php
@@ -45,10 +45,7 @@ class EbookDownload{
 			$this->UserAgent = null;
 		}
 
-		// The `IpAddress` column expects IPv6 address strings.
-		if(is_string($this->IpAddress) && filter_var($this->IpAddress, FILTER_VALIDATE_IP, FILTER_FLAG_IPV4)){
-			$this->IpAddress = '::ffff:' . $this->IpAddress;
-		}
+		$this->IpAddress = Formatter::ToIpv6($this->IpAddress);
 
 		if($error->HasExceptions){
 			throw $error;
@@ -86,4 +83,26 @@ class EbookDownload{
 					and Created < ?
 			', [$startDate, $endDate], EbookDownload::class);
 	}
+
+	/**
+	 * Gets the count of `EbookDownload` objects by the given IP address newer than the given created time.
+	 *
+	 * @param string $ipAddress The IP address to search for.
+	 * @param DateTimeImmutable $startDateTime The minimum creation timestamp (inclusive).
+	 * @return int The total number of `EbookDownload` objects matching the criteria.
+	 */
+	public static function GetCountByIpAddressSince(?string $ipAddress, DateTimeImmutable $startDateTime): int{
+		if(!isset($ipAddress)){
+			return 0;
+		}
+
+		$ipAddress = Formatter::ToIpv6($ipAddress);
+
+		return Db::QueryInt('
+				SELECT count(*)
+				from EbookDownloads
+				where IpAddress = ?
+					and Created >= ?
+			', [$ipAddress, $startDateTime]);
+	}
 }
diff --git a/lib/Enums/HttpCode.php b/lib/Enums/HttpCode.php
index 20d58acd..66698034 100644
--- a/lib/Enums/HttpCode.php
+++ b/lib/Enums/HttpCode.php
@@ -24,6 +24,7 @@ enum HttpCode: int{
 	case Conflict = 409;
 	case Gone = 410;
 	case UnprocessableContent = 422;
+	case TooManyRequests = 429;
 
 	case InternalServerError = 500;
 	case ServiceUnavailable = 503;
diff --git a/lib/Formatter.php b/lib/Formatter.php
index 3a14292a..b30c8d5f 100644
--- a/lib/Formatter.php
+++ b/lib/Formatter.php
@@ -149,6 +149,17 @@ class Formatter{
 		return $output;
 	}
 
+	/**
+	 * Format a valid IPv4 address to IPv6. Other strings are unchanged.
+	 */
+	public static function ToIpv6(?string $ipAddress): ?string{
+		if(filter_var($ipAddress, FILTER_VALIDATE_IP, FILTER_FLAG_IPV4)){
+			return '::ffff:' . $ipAddress;
+		}
+
+		return $ipAddress;
+	}
+
 	/**
 	 * Format a float into a USD currency string. The result is prepended with `$`.
 	 *
diff --git a/www/ebooks/download.php b/www/ebooks/download.php
index 0819138a..87e3a683 100644
--- a/www/ebooks/download.php
+++ b/www/ebooks/download.php
@@ -7,6 +7,11 @@ $downloadUrl = null;
 $downloadCount = HttpInput::Int(COOKIE, 'download-count') ?? 0;
 $source = Enums\EbookDownloadSource::tryFrom(HttpInput::Str(GET, 'source') ?? '');
 
+$shortDownloadLimit = 10;
+$shortDownloadTime = NOW->modify('-30 seconds');
+$longDownloadLimit = 100;
+$longDownloadTime = NOW->modify('-1 day');
+
 // Skip the thank you page if any of these are true:
 // * The user is logged in.
 // * Their `download-count` cookie is above some amount.
@@ -40,11 +45,17 @@ try{
 	/** @var string|null $userAgent */
 	$userAgent = $_SERVER['HTTP_USER_AGENT'] ?? null;
 
-	try{
-		$ebook->AddDownload($ipAddress, $userAgent);
-	}
-	catch(Exceptions\InvalidEbookDownloadException){
-		// Pass. Allow the download to continue even if it isn't recorded.
+	if(!isset(Session::$User)){
+		// Check for excessive downloads.
+		$shortDownloadCount = EbookDownload::GetCountByIpAddressSince($ipAddress, $shortDownloadTime);
+		if($shortDownloadCount > $shortDownloadLimit){
+			Template::ExitWithCode(Enums\HttpCode::TooManyRequests);
+		}
+
+		$longDownloadCount = EbookDownload::GetCountByIpAddressSince($ipAddress, $longDownloadTime);
+		if($longDownloadCount > $longDownloadLimit){
+			Template::ExitWithCode(Enums\HttpCode::TooManyRequests);
+		}
 	}
 
 	if($skipThankYouPage){
@@ -56,6 +67,13 @@ try{
 			throw new Exceptions\InvalidFileException();
 		}
 
+		try{
+			$ebook->AddDownload($ipAddress, $userAgent);
+		}
+		catch(Exceptions\InvalidEbookDownloadException){
+			// Pass. Allow the download to continue even if it isn't recorded.
+		}
+
 		// Everything OK, serve the file using Apache.
 		// The `xsendfile` Apache module tells Apache to serve the file, including `not-modified` or `etag` headers.
 		// Much more efficient than reading it in PHP and outputting it that way.