Before history was recorded upon writing, nowadays however, the historian has much more sources to his disposition. With all the upgrades and technologies of the 21st century, the internet has become the center of all information; search through a book is not totally necessary. With inventions such as Google where you can simply type a word and a ton of files appear, the historical evidence is more accessible. Doing research in a history book can take days or weeks, but with technology it is much easier and less annoying for an historian to find evidence.
Future historians are always going to have access to books as their primary source when they are doing a research. However the “tweets” can become their secondary sources, because they are taking into count things withnessed by people the minute the action (or event) happened.