I am not here to talk about guidebooks. However, I do believe you should be reading prior to and during your trip. In particular you should be reading history books.
Most guidebook will have a short section on the history of whatever place they cover. The problem is, your average Wikipedia article will usually give you more information than the history section of a guidebook. I don’t fault guidebooks for their history coverage though, because they aren’t in the history book business.
To really understand a culture, you have to understand its history. Food, dancing, language and religion are all important, but sort of meaningless without the background that history can provide.Example: I was visiting Yogyakarta, Indonesia visiting the two great temple complexes of Prambanan and Borobudur. Prambanan is a Hindu temple. Borobudur is a Buddhist temple. Modern Indonesia is mostly Muslim. Connecting those dots gives you a sense of the sweep of history that has shaped Indonesia. It explains why Bali is Hindu. It can help explain why Most of SE Asia is Buddhist. Most importantly, it lets you see culture as a dynamic process, not as something static. Culture today is just a snapshot and it is different from culture 100 or 1,000 years ago.
To really get a sense of history, you need to read long form content. The internet, while great at some things, is horrible at others. You wont get a rich understanding of the history of a place by reading a 500 word blog post. You at least need to read a book, complete with footnotes and a bibliography.
What history you should read is really up to you. Not all history needs to be ancient history. Try to fill in the gaps you know nothing about. Before I visited Japan I read up on post war Japan, in particular the great Pulitzer Prize winning book Embracing Defeat: Japan in the Wake of World War II. I figured learning about pre-Miji Japan was great and all, but to really understand Japan today you need to understand the years after WWII…which I knew nothing about.
Another great book I read was the biography of Douglas McArthur, American Caesar: Douglas MacArthur 1880 – 1964. McArthur’s life story covered the Philippines, Australia, Papua New Guinea, Japan and Korea. The story of the war in the Pacific, especially PGN campaign, is little known in the US.
When I was in the Middle East I read up on another big, important chunk of world history I new little about. The spread of Islam and the rise of the Caliphate. The Great Arab Conquests: How the Spread of Islam Changed the World We Live In was a great introduction in the spred of Islam in the 7th Century.
After visiting Singapore in 1999 I read From Third World to First : The Singapore Story: 1965-2000 the autobiography of the founder of modern Singapore, Lee Kwan Yew. (Hint: always take an autobiography with a grain of salt)
You owe it to yourself to have more than a cursory understanding of wherever it is you are visiting. What part of history and what book you read is really secondary to the attempt to understand how the place you are visiting came to be. If you must choose, grab the history book before you grab the guidebook. You won’t regret it.