I bought this book as an additional aid to a University class that I'm taking. The message of it is very simple, and likely things that people have always noted before. I don't think that it really took a Doctor to write it. Anyone that thinks, feels, and observes their mate, can figure this stuff out. That being said, some people are oblivious, or perhaps don't realize something until it's said and the light bulb goes off.
People have different things that makes their heart tick. Each of us respond to love in different ways, and have different needs. The author boils seeing love displayed in five different ways, and when one identifies the language of love their mate speaks, then it will be easier to please them. For quite sometime I have been aware that I'm a "gift" person. I tend to give gifts to those I love, because I myself feel the most love from receiving them.
Keep in mind this book is highly Bible based. Some references are very obvious, others are going to be known only by those familiar with the Bible. Some Biblical passages are discussed by the author through the words "an ancient sage once wrote." Nearly everything quoted, including C.S Lewis, is because of their spiritual content. However, even if you aren't one for believing in the Bible, the passages used are still of value since they display common sense.