Also, how accurate can books about something as old as the Middle Ages be? How do you measure the accuracy of a book. On Wikipedia, I've seen many facts that were backed up by books but what are the sources of the book itself?
I am not sure how to answer your second question. That is a super big question, one of the main questions in history. I think one possible start would be, depending if you can get your hands on it a book like:
Backman, Clifford, The Worlds of Medieval Europe, Oxford 2004.
The reason: This book covers different aspects of the middle ages and each time suggests also some typical authors for the time, so when you read a chapter you will afterwards also get recommendations for some sources.
Otherwise you could also start with a real handbook, like we would do at the university, if you feel comfortable with that. For the English speaking world an example would be "The New Cambridge Medieval History" it has 7(8) volumes and is published by some of the leading scholars in the field, because it is not a monograph you can just select chapters in the volumes about topics you care about and start from here. Contrary to some other publications this books do not have as much references and are more of an overview of the topics.
Otherwise please look in the list already in this subreddit: https://www.reddit.com/r/AskHistorians/wiki/books/europe#wiki_middle_ages
https://www.reddit.com/r/AskHistorians/wiki/books/historiography