I would buy it. I'm guessing it's too expensive to do that though. It feels like a waste that I will know all about this CPU and the inbuilt language but not have a real platform to develop on when I eventually finish the book. However I still appreciate it a lot.
You can create the CPU and a computer around it for a limited amount of money yourself with an FPGA board. For example low-cost starter kits using a Cyclone V FPGA (or comparable Xilinx chips) even have enough internal memory to host ROM, RAM and the screen RAM. You can also go for even cheaper chips (I.e. Cyclone III) and boards with sufficient SD-RAM (which should be the case for all modern fpga boards), however this requires you to program or integrate a simple SD-RAM controller in your HDL design and manage potential wait states in addition.
It will (almost certainly) never be produced in quantities that would drive the prices down to something that is reasonable from a marketing perspective, which means that it will (almost certainly) never be produced, period.
But it can certainly be implemented in an FPGA -- lots of people have done that. Heck, I've done it. The architecture is so simple that it's a matter of a couple of hours to get the entire thing implemented on a pretty cheap FPGA development board. What you lack at that point are the peripherals -- the keyboard interface and the driver for the screen. Neither of those are too difficult to design and implement, at least not to the level that they are needed to replicate the behavior of the CPU Emulator.
This would be fine for something that you would consider using for an embedded system, but there are much better alternatives for that available so that would lose it's luster pretty quickly. What could capture your interest for quite some time is a general purpose computer ala the ones that were available back in the 1970s and 80s. For that you need some additional capabilities, such as the ability to store programs on non-volatile media (i.e., a "disk" of some kind). Something like an SD card would likely be ideal. Then you need to have a way of loading programs from disk into memory and executing them. This would likely take the form of turning some of the program ROM into RAM, but it would also require modification to the hardware design to enable the CPU to write to the ROM memory space. This could actually be accomplished with the same instruction set by using memory-mapped IO to do it. Finally, you would need to augment the OS with classes to manage all of this as well as some kind of terminal program that gives you some kind of command prompt to work from.
Once you had all of that, you have a functioning computer -- provided you write your programs on a different machine, compile them on that machine, and then store the .hack files on the SD card and insert it into the machine. The eventual goal should be to make this new computer truly stand alone. So you need an editor to let you edit text files on the computer itself and you need a compiler that runs on the computer itself. A potentially good path to get there is to first implement a Jack interpreter that can run on the Hack hardware (that interpreter would initially be written in some other language and compiled for the Hack using a tool chain running on some other computer).
It would not be a trivial undertaking, but it seems like a very doable one and it would be immensely educational.
It feels like a waste that I will know all about this CPU and the inbuilt language but not have a real platform to develop on when I eventually finish the book.
The thing to keep in mind is that the really valuable things you have learned are not about this particular CPU or the Hack/Jack language. It's the concepts of how computers work from top to bottom and those things are rapidly transferable to other platforms.