The 5 Love Languages by Gary Chapman has been on my list of "to reads" for the past couple of months. After reading it though, I wish I had spent the day (yes, it is a 'read in a day' kind of length) to read it and understand more about it all. The interesting part about The 5 Love Languages is that it does necessarily have to apply to a romantic relationship. The book is definitely centered around those and marriages, but learning the languages can help you with other relationships. Thus, I would put The 5 Love Languages on my recommended list - a must read for everyone who is looking to learn more about themselves and others.
You can also learn about your love language online - if you don't feel like reading.