The Spectrum manual was good, with the usual programming guide and a breakdown of the memory map and system variables, but the Jupiter Ace manual also included example circuits so you could build your own peripherals...
The manual was great for its time, a little dated now but still a decent read [2]. It was reprinted in 2018 and still available [3].
[1] https://en.wikipedia.org/wiki/Jupiter_Ace
[2] https://archive.org/details/vickers_steven_jupiter_ace_4000_...
[3] https://www.amazon.com/Jupiter-ACE-Manual-Anniversary-Progra...
Also wrote the (Jupiter and Sinclair implementations of the) languages too!