Index: book/ch05.xml
===================================================================
--- book/ch05.xml	(revision 12449)
+++ book/ch05.xml	(working copy)
@@ -545,6 +545,22 @@
         </varlistentry>
       </variablelist>
 
+      <warning>
+	<para>Do not attempt to modify the transaction using the hook
+	scripts. A common example of this would be automatically
+	setting properties such as eol-style or mime-type during the
+	commit. While this might seem like good idea, it causes
+	problems. The main one is that the client which made the
+	commit is no longer current with what is in the repository
+	(because the hook script modified it), but there is no way to
+	inform the client that it is out-of-date.</para>
+
+	<para>Instead of attempting to modify the transaction, it is
+	much better to <emphasis>check</emphasis> the transaction in
+	the <filename>pre-commit</filename> hook and reject the commit
+	if it does not meet the desired requirements.</para>
+      </warning>
+
       <para>Subversion will attempt to execute hooks as the same user
         who owns the process which is accessing the Subversion
         repository.  In most cases, the repository is being accessed