As fields are implicitly private, why there is often explicit declaraion used in the books, articles etc.?
Sometimes explicit is better than implicit, and this is even more so when you are writing educational material. For people who do not know or cannot remember the rules for the default access levels it is one less thing for them to be concerned with when reading the code.
Related Question
The default access for everything in C# is "the most restricted access you could declare for that member".