<?php
$products = array(
    'CATEGORY' =>
    array(
        0 => 'book',
        1 => 'book',
        2 => 'desk',
    ),
    /* FYI: Other keys have been removed for conciseness */
    'DESCRIPTION' => 
    array(
        0 => 'Bar',
        1 => 'sdfadasfdasfas',
        2 => 'Barrrr',
    ),
);

$newProducts = array();
$newProducts['CATEGORY'] = array();
foreach ($products['CATEGORY'] as $id =>$cat) {
    $newProducts['CATEGORY'][$cat] []= array('DESCRIPTION' => $products['DESCRIPTION'][$id]);
}

var_dump($newProducts);
?>

array(1) {
  ["CATEGORY"]=>
  array(2) {
    ["book"]=>
    array(2) {
      [0]=>
      array(1) {
        ["DESCRIPTION"]=>
        string(3) "Bar"
      }
      [1]=>
      array(1) {
        ["DESCRIPTION"]=>
        string(14) "sdfadasfdasfas"
      }
    }
    ["desk"]=>
    array(1) {
      [0]=>
      array(1) {
        ["DESCRIPTION"]=>
        string(6) "Barrrr"
      }
    }
  }
}