This is the (almost) full reference of the MSX2 system as published by ASCII Corporation in 1987. I converted it to text files in 1997 (the OCR used consisted of my eyes and my fingers) and now here it is in markdown format for easier browsing and reading. There's also the BASIC KUN (also known as Turbo-BASIC) compiler reference as a bonus.
These are the original text files as I typed them back in 1997, but with tabs converted to spaces and with some typos fixed. Unless you plan to read these directly in your MSX you probably prefer to read the formatted Markdown files instead.
Note that the file sizes listed on the index file aren't valid anymore after the corrections. Also, that file mentions that chapter 1 is compressed because the files were initially distributed in a 2DD disk.