You can't read stuff on a bookshelf; you have to take it out and put it on the table to open it. (Sure, you can read a book on a couch, but this is the computer we're talking about - it has certain rules.)
You can take out as many books from the bookshelf as you like, as long as they fit on the open table.
If you run out of table space, you need to make more space on the table, and the only way to do that is to put books back on the shelf.
If you're trying to open too many books at once with too little table space, the only thing you can do is constantly go back and forth between the bookshelf and the table, open the book, find your place, etc., read a word, take it back, open the previous book, write it down, etc., and it's going to take you way longer to do the same work.