There are good books on cat behavior, but I don't think anyone has written in much depth on cat behavior when they lost a companion. I assume you have considered getting Sookie a kitten? A lively little male will help fill her days.
One more vote to "try another kitten/cat"
Melinda, sorry I don't have any advice to help Sookie, you are already giving her extra love and attention and thats all I could have recommended, its just going to take her some extra time. Animals grieve just as we do and we are all different in just how much time its going to take..sending her some purrs and scritches xoxoxo
When Queen Victoria died at age 17 1/2, followed closely by Cerridwen Blythe at 16, we watched Calamity Jane who was Victoria's womb-mate.
Gave her extra loving and treats, but she did not do well as a solo kitty. So, one of y co-workers had kittens ready, which is how we got Silver Kestrel and then her sister Hildiekatt a year later.