diff -urp --strip-trailing-cr ../category/category.install ./category.install
--- ../category/category.install	2009-08-05 06:58:56.000000000 +0200
+++ ./category.install	2009-09-12 16:24:31.000000000 +0200
@@ -34,6 +34,19 @@ function category_install() {
   
   variable_set('category_behavior_category', 'category');
   variable_set('category_behavior_container', 'container');
+
+  // Explain about the wrapper modules installation, and the need of category_legacy import
+  // of pre-existing data, where applicable.
+  $types = array();
+  foreach (array('book' => 'book', 'taxonomy' => 'vocabulary') as $type => $table) {
+    if (db_table_exists($table) && db_result(db_query("SELECT count(*) FROM {". $table ."}"))) {
+      $types[] = $type;
+    }
+  }
+  drupal_set_message(t('The Category module was successfully installed. To use the new features properly with other modules depending on Book and/or Taxonomy, you should <a href="!path">install</a> Taxonomy and/or Book wrappers prior to enabling such modules. Beware that wrappers are significantly changing the way how books and taxonomies work.', array('!path' => url('admin/content/category/wrappers'))));
+  if (!empty($types)) {
+    drupal_set_message(t('Your site already have some %types data, needing to be converted using the category_legacy module (after the corresponding wrapper(s) have been installed), to become compatible with the newly installed Category system.', array('%types' => implode(t(' and '), $types))), 'warning');
+  }
 }
 
 /**
diff -urp --strip-trailing-cr ../category/INSTALL.txt ./INSTALL.txt
--- ../category/INSTALL.txt	2009-08-05 06:52:53.000000000 +0200
+++ ./INSTALL.txt	2009-09-12 16:53:49.000000000 +0200
@@ -51,31 +51,39 @@ Installation
    enabled, and continue to use your book data, if you wish (but as with
    taxonomy, it is recommended that you disable book).
 
-7. Go to 'Administer -> Permissions' (admin/user/permissions) to configure
+7. If you're going to install any new taxonomy and/or book dependent modules,
+   it's highly recommended to enable them only after you've installed Category,
+   as well as corresponding wrapper module(s), to avoid the need of
+   category_legacy import. Some modules (such as Forum, Image Gallery,
+   Simplenews and others) are known to create taxonomy structures automatically
+   as a part of their installation process, in a way that's not always fully
+   compatible with the category_legacy import.
+   
+8. Go to 'Administer -> Permissions' (admin/user/permissions) to configure
    access privileges for this module.
 
-8. Settings for this module are basically located at 'Administer -> Categories'
+9. Settings for this module are basically located at 'Administer -> Categories'
    (admin/content/category), but you'll notice that nearly all of the wide
    range of options are actually set on per-container or per-node-type basis,
    so you need to visit 'Administer -> Content types -> edit' (admin/content/
    node-type/your_node_type), and especially your container's edit page, to
    make all the choices.
 
-9. Go to 'Administer -> Categories' to start managing your categories and
+10. Go to 'Administer -> Categories' to start managing your categories and
    containers. You can create new categories and containers from here, or
    you can go to 'Create content -> Category' or 'Create content -> Container'.
    As you'll be creating your categories and containers, you can adjust their
    behavior to your needs through various settings found in the 'Category
    information' or 'Container information' boxes.
 
-10. You can assign content to categories by configuring your containers to be
+11. You can assign content to categories by configuring your containers to be
    associated with various node types, and then creating or editing nodes of
    those types.
    Note: content with assigned categories will only have its categories
    displayed, if the taxonomy wrapper is installed, and if category_display
    is not set to hide them.
 
-11. You may create customized lists of content based on categories, if you
+12. You may create customized lists of content based on categories, if you
    have installed the Views module (available at http://drupal.org/project/views),
    and also use them for your categories through the category_views module.
    To use Category data with Views, you need to install the taxonomy wrapper,