ZX Spectrum Next Assembly Developer book
The book you need to get the maximum out of a Spectrum Next is here, in its second revision, fully printed. A work of technical mastery by Tomaz Kragelj, this is a must-read for anyone wishing to code in Assembly for the Next.
Buuuuut, if you don’t want a printed, luxurious book, well… There’s always the PDF format made freely available by the kind Tomaz. It’s your choice…
The printed book: https://www.lulu.com/shop/tomaz-kragelj/shop/tomaz-kragelj/zx-spectrum-next-assembler-developer-guide/paperback/product-7d2y85.html
And the PDF download: https://github.com/tomaz/zx-next-dev-guide