If you are trying to work out where to advertise online, the answer is simpler than you may think.
The best place you can advertise your goods or services is on sites with endemic content. This simply means that if you are selling books, then a site about books is generally the best place to put your ads. There are no guarantees this will be the case however, so you shouldn’t put all of your eggs in one basket, so to speak. Therefore don’t put all of your advertising budget on just one site about books if you can help it, but spread it across many so that you can compare and contrast the results they bring, and then you can reallocate your budget to the best site once you are sure.
If there are no endemic sites out there for you to buy into, then Google AdWords is always a good choice, as with all Google products, it is easy to use and the results you get from it can be pretty good if used correctly.
There are other ad networks out there for you to use, but unless you have an ongoing and reasonable sized marketing budget available, most ad networks will not care about your campaign at all once it goes live, and will do the bare minimum to make sure they earn your money. Of course, they will all say differently and try their hardest to get their hands on your budget, but unfortunately it’s all malarkey.
Wherever you advertise, you should always make sure of one thing – that you get an ad slot which is “above the fold” as this means that your ad will be seen as soon as a page loads, without any scrolling. Statistically speaking, above the fold ads always perform much better than those below the fold.