You have too many plot lines in this story to really cover any of them well. This relegates you to constantly jump around between characters and not focus in any one case too much.
In the beginning you also suffered with a great deal of telling the reader what was happening instead of showing us what was going on.
Second, you have strange happenings that might make sense to you as an author going on that the reader is given no hints about even vaguely going on - you have people who can mystically get the information from a book that purges itself after five entries....
Then we get to the furthest I've read - the issue with Hermione's mom going into shock and neither Harry or Hermione noticing at all is frankly ludicrous. Also you have them becoming exhausted after using very little magic in their world when they were doing much more in a day in "Reality".
The most important thing an author has to remember is internal consistency. Rowling fouled that up for three books, if someone didn't ghost write the last three with her notes helping them.
Unfortunately, it seems you've done the same thing. Harry and Hermione did several feats of magic in the real world that they seemingly couldn't accomplish in their own world saturated with magic.
Finally, you've got a man who was seemingly not intelligent enough to brew high end potions doing a ritual that caused all of Britain's potions to foul up... Thoroughly unbelievable.
This isn't intended as a flame, rather I'm hoping you'll improve your work. It began interestingly enough, although the "Dorothy" moment was a poor way of moving them between dimensions.
Unfortunately your latter chapters became much less interesting.