I have a custom post type "books" and a custom taxonomy books-categories. wp_insert_post() works but when i choose a category and submit the form the post displays ut the category isn't selected. Can anyone help me please?

if( 'POST' == $_SERVER['REQUEST_METHOD'] && !empty( $_POST['action'] ) &&  $_POST['action'] == "new_post") {

    // Do some minor form validation to make sure there is content
    if (isset ($_POST['title'])) {
        $title =  $_POST['title'];
    } else {
        echo 'Please enter a  title';
    }
    if (isset ($_POST['description'])) {
        $description = $_POST['description'];
    } else {
        echo 'Please enter the content';
    }
    $tags = $_POST['post_tags'];

    // Add the content of the form to $post as an array
    $new_post = array(
        'post_title'    => $title,
        'post_content'  => $description,
        'post_category' => array($_POST['cat']),  // Usable for custom taxonomies too
        'tags_input'    => array($tags),
        'post_status'   => 'draft',           // Choose: publish, preview, future, draft, etc.
        'post_type' => 'books'                //'post',page' or use a custom post type if you want to
    );
    //save the new post
    $pid = wp_insert_post($new_post); 
    //insert taxonomies      
}