Can a professor force you to buy a book, for use in his exam? I'm a first year law student in the UK. For our EU Law class, our professor declared that we must bring an unannotated physical copy of Blackstone's EU Treaties &amp; Legislation 2018-2019 to our exam and use it. Thus we must buy it.

Our Prof admitted that these laws are free online. But we obviously can't bring our own printouts to the exam, as the invigilators can't check each student's printout. 
The professors for my three other classes don't require us to buy any book for the exam, as they'll provide us with the legislation for the exam. 


Thus why don't the EU Law professors do the same? I wonder...laziness? 


Our professor ought to provide the legislation, because the book


is a waste of money. Statues and legislations change yearly, and this book's going to be outdated. In fact, scroll down the Amazon page, and you'll see that the book has a new edition  published yearly. 
is too eco-unfriendly! 1 sheet/2 pages  x 696 pages x 200 students = 69,600 sheets! And the professor has been requiring this book for many years.
may be unaffordable. It's £14.99 + £3 VAT, which is under the poverty threshold. 


I emailed our professor many times, but he never replied. I prefer to stay confidential, and not to confront him or the administration face-to-face. I emailed the administration, but they just keep repeating the professor's order to buy the book.
As user2768 already pointed out in a comment, the question is not whether the prof can force you (no, they can not), but if it is reasonable / fair to require a specific book for an exam. 

The answer can depend on many factors:


Is the professor the author of the book? If yes, I would see an issue because this would boost their personal income. This does not seem to be the case in your situation.
Can the exam be completed without the book? E.g. in technical fields, it might be helpful to have a formulary, in languages a dictionary, etc. In most cases, it it just reasonable to use such a book because otherwise you would have to memorize too many facts / formulas, etc. 
Is the book a pre-requisite for doing the exam? I did not study law, but from my understanding, one of the natures of law is to derive conclusions based on written text (especially laws). Therefore, it is impossible to do the exam without access to the original law text. So in your specific case, I would say the professor has to require the book, otherwise it would be impossible to do a proper exam. 
Is the price / effort unseasonably high? In your case the short answer is "no". In fact it is extremely cheap. 
Are there reasonable alternatives to a printed book? Electronic versions of the text would provide an unfair advantage over students using he paper version since they are searchable, in addition it is difficult to guarantee the sanity of the device. One solution might be to provide each student with a computer holding a local copy of the law text and having the internet connection disabled during the exam. The university would need a tremendous infrastructure for that. 


To make a long story short: Buy the book. And don't throw it away after one year ;-). Maybe you can hand it over to a student next year. 

One additional idea: If you are having some sort of students association: Ask them to organize a bookshelf for such kind of literature.