Hi Murray,
Both books were targeted to handle the release of Version 11. Given the scope of each numbered release of Mathematica and the Wolfram Language it is tough to encapsulate all of the changes from each version. Every single chapter was changed, though not always significantly. We workshopped and fixed a number of the exercises ( our wonderful Wolfram Community had pointed out a number of issues in both books). Sometimes its small things, like when a function is superceded by a new function that does the old functions work, plus a little more. Other times it's a large addition to our Machine Learning capabilities. You may have seen Stephen's introductory blog post to the new edition of EIWL: Machine Learning for Middle Schoolers.
One thing to note is that both editions changed to physically less daunting volumes. The new Hands on Start (HOS) is now 7x10 and easier to carry. The new Elementary Introduction (EIWL) was printed on a much lighter weight paper, again to keep the volume lightweight and easy to transport.
Substantively, you might take a look at this Blog post for the new HOS: Meet the Authors of Hands-on Start to Wolfram Mathematica, Second Edition. It also includes an index, which the first edition did not have.
EIWL had significant advances in our product stack detailed, for example interacting with datadrop, datarepository, and the Wolfram Cloud.
Happy to answer any further questions.
Jeremy Sykes
Publishing Assistant