how to understand these books better?

I think if you really want to understand everything... you need to be patient... if there is a word that you do not understand... then try consulting the dictionary... that will be a great help... it would also be great if you read the book in your native language... then read it in English... that way you will understand most of the things they are talking about...

It is necessary to look up words that we do not know in a dictionary. But then the flow of reading may be disturbed. One thing that could be done is keep noting the words, in a piece of paper kept along with the book. You could later use uit as a bookmark too, if possible, with the sentential context in which they are used. Then look 'em all up in one go at some time in the day.If you can make it a part of your routine to look up say, 5 words per day, you would be accumulating quite a lot in a day. But as for me I would try to get at an approximate meaning of the words that comes up from the context. Then when I look up it turns out to be more or less correct, then you feel a sense of achievement. These techniques serve to pep up our reading spirits.

My only advice for words you don't know would be to look them up in a dictionary so that you know them in your own language. But I have to warn you, in the Harry Potter books the author has made up a lot of words that don't have any real meaning. They are just words that the wizards in the book use but aren't real words used in real life. If I were you, I might try to read them in my original language first, if possible, and then re read them in english. That way if you don't understand a few of the words you will still be able to understand what the whole book is about.