Why not get one as wedding present for them!
I don't agree with everything they say, and there are some generalisations ~ but I do think they are worth reading!

'Men Are from Mars, Women Are from Venus: A Practical Guide for Improving Communication and Getting What You Want in Your Relationships' by John Gray
http://www.amazon.com/Mars-Women-Venus-Communication-Relationships/dp/006016848X
'Men, Women and Relationships : Making Peace with the Opposite Sex' by John Gray
http://www.amazon.com/Men-Women-Relationships-Making-Opposite/dp/B0002OUQN0/ref=pd_sim_b_img_4
'Mars and Venus Together Forever: Relationship Skills for Lasting Love' by John Gray
http://www.amazon.com/Mars-Venus-Together-Forever-Relationship/dp/0060926619/ref=pd_bxgy_b_img_b
The Venus & Mars site is here:
http://www.marsvenus.com/
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'Why Men Don't Have a Clue and Women Always Need More Shoes: The Ultimate Guide to the Opposite Sex' ~ Barbara & Allan Pease
http://www.amazon.com/Dont-Have-Women-Always-Shoes/dp/0767916107
'Why Men Don't Listen and Women Can't Read Maps: How We're Different and What to Do About It' ~ Barbara & Allan Pease
http://www.amazon.com/Dont-Listen-Women-Cant-Read/dp/0767907639/ref=pd_bxgy_b_img_b
Allan and Barbara Pease site here:
http://www.peaseinternational.com/

"The secret of success is constancy to purpose" - Benjamin Disraeli.