This is unfortunate on lots of levels. Firstly, school are very very hard up for resources. It may well be that they can't afford to buy all the books they would like the kids to have. That said, they cannot REQUIRE parents to buy stuff for the curriculum, although they can RECOMMEND - quite different and unfortunate that they sound like they have made it sound like it is required, unless that is just an interpretation of their wording.
Clearly, some people won't be able to afford the stuff and so the school has to work on that basis and make sure no one is disadvantaged which will mean the school buying some books or photocopying or using online resources. If they cannot buy the books as a school,N they will not be able to teach in a way which requires all children to have their own set of books - they just won't be able to.
I would imagine that either the letter doesn't say the books are required but strongly encourages it, or some kind of junior person has written the letter and sent it without the Head seeing it first. If the latter, the school needs this pointed out to them and needs to send another letter making clear there is no compulsion and explaining if and how the books will be used.
If it is the former and the school are strongly recommending getting the books - then you need to decide if you can and will. Personally, in these days if cut backs, when schools ask parents to buy things like books, I'm of the view that if you can afford it, you just accept that this is the sorry state of school funding and get on and buy it. If you can't afford it, of course you just can't. Stating things are a requirement aside, which was obviously and is obviously wrong, I think we need to understand that schools will and need to make more requests for parents to pay for things - we might not like it, but that is the reality. As long as they only request and have a plan in place so all can access the curriculum, it's just going to be the way of the future. And unless those who can pay do, rather than deciding on principle not to pay or just thinking they will slip under the net and not pay because there is no way to force payment, the what school can provide and the options open to schools in the ways they teach, will just become more spartan, narrow and grey.
I think we're at the point where if you can pay, you need to. Lots of people will think this is unfair that they are paying when others can say they can't afford it and avoid paying, if actually they can afford it - that is simply a reality, but making payment optional for curriculum based activity is the only way to do it to ensure all, including those who genuinely can't afford it have access to the basics. Unfortunately there will probably be more and more of a two tier system with the basics provided for all and everything beyond this only being provided to those who can and will pay.