Oh fountain of wisdom - In the book 'Programming the TI-83 plus/TI-84 plus., on page xv11 there is a mention of 'A rudimentary TI-BASIC tutorial. and 'A guide to advanced TI-BASIC optimisations'
Are these books still available ?
Many years ago I learned that if you became interested in a subject, it was wise to get every book available on the subject, since any one of those books is liable to contain invaluable information which none of the others had.
Second question. If a programme is written on and for a TI-83, what are the conditions for it to run successfully on a TI-84 plus ?
I suspect that the same operating system would be the only condition.

Excellent questions, and sharp reading. Both of those references are short tutorials (short relative to my book, anyway), and available freely here on Cemetech. Here's they are:
:: The rudimentary tutorial, which I feel is far inferior in quality to my book and covers the same material: TI-83/+/Silver BASIC Tutorials
:: The optimization guide, The 1337 Guide to TI-BASIC. Chapter 10 of my book teaches almost everything in this plus extra optimization tips.

Also, I have moved this topic to the subforum for the book.

To the best of my knowledge, any code for a TI-83 should be forwards compatible with the 83+ and 84+ series. Backwards compatibility isn't always guaranteed though.

Sounds reasonable. Thanks for that.