I turned 70 in June 2016 and filed for and have been receiving full SS benefits.
My wife turns 66 in March 2017.
Is she "grandfathered in" to be able to file and suspend her own benefits at that time and file to receive spousal benefits from me only?
She has prepared a letter like the one you suggest in your book. Should she send it? or make an appointment?
Yes, your wife is grandfathered under the new deeming law (https://www.ssa.gov/planners/retire/deemedfaq.html), but she would not want to file and suspend her own benefits. If she did, it would prevent her from being able to receive a spousal benefit on your record.
What she should do instead is file a restricted application for spousal benefits only effective with the month she reaches age 66. She'll want to make sure that the application contains a remark such as 'I wish to restrict the scope of this application to spousal benefits only.', or 'I wish to restrict retirement benefits on my own record from the scope of this application.' She can then let her own benefit rate grow until she reaches age 70.
I don't know what kind of a letter you're referring to, but it sounds unnecessary. She simply needs to file a spousal application as described above, which she can either do online at ssa.gov, or by calling or visiting a Social Security office.