Depending on how old your wife is she may be going through premenopause. Although my wife still enjoyed sex she was finding it hard to achieve orgasm. I posted on TAM and was directed to Official Web Site of John R. Lee, M.D.
and told to get his book What Your Doctor May Not Tell You About Menopause
. He also has a book for younger women (age 30-50) What Your Doctor May Not Tell You About PREmenopause
If her difficulty is because her attraction to you is gone then I would suggest you look at Married Man Sex Life
These are two things that come to mind right off the bat.