Personally I love having a real, physical guidebook in my hands to find out more about a destination. I like being able to flick from chapter to chapter in the wrong order and put in bookmarks (well scraps of paper) on the pages that interest me. My favourite guides are by Lonely Planet* followed by Rough Guide. I also like the Bradt guides especially for Africa. Most books highlight popular sights and experiences and some will give you rough itineraries with time frames to help you plan. They all give you detailed information about thousands of places and all have trip planning information online too.
Most countries have an online tourist board to promote their country to tourists and these can be a good place to start to find out about a particular destination. Many States or Provinces within a country also have their own website which can be really useful if you want to stay local and find out what's close by. As well as an online presence even the smallest little place will often have a tourist information office where you can go and find leaflets, maps and helpful staff to tell you what's going on locally.
Travel Blogs & Newspapers
There are thousands of travel bloggers online with stories of their travels from all over the world. Many travel companies also use travel blogs to give information to their customers. Personally I like a good travel blog and especially when you find the authors you connect with as they can be really helpful in guiding you to new places you might really like. I also really like the Guardian and the Independent newspapers for their travel sections which are full of inspiration and information.
As we know Facebook has groups for everything and you'll be amazed at how many groups there are about travel and destinations. There are some really specific ones such as 'French Campsites' which are full of great tips so be specific in your searching. The benefit of these groups is that they are often really up to date as people are often posting from these destinations. You also have the chance to ask questions and hopefully have them answered by people who have been there and done it! Finally you can search back through the group's history to find previous posts about places that interest you.