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.
