I did consider including a 'traditional' index, but I came to the conclusion that it wouldn't add much that isn't provided by the index of Maple Notation and the Table of Contents. Please let me know if your students report difficulties finding information in the book. If the lack of a traditional index causes a widespread problem I will produce one and make it available for download (and add it to any future editions).
Should there be a chapter on statistics? I tried to limit the book to material that is central to understanding Maple, and material that a large majority of users are likely to need. I don't think statistics would fall into the first category, but one could argue that a large majority of users will need it, especially since many science undergraduates take one or more statistics courses in their first year. On the other hand, I don't know how widely Maple is used for statistics. There wouldn't be much point adding a chapter on this if statisticians all prefer R or minitab.
Finally, I think single ditto operators are relatively harmless in simple situations. Also, in my experience, Maple users tend to use shortcuts such as this even if they are advised not to, especially if they can't see an immediate problem with what they are doing. If I took a 'hard line' approach, and suggested that ditto operators should be avoided, then I'm afraid my advice might be ignored. Instead I decided to use ditto operators in some of the shorter examples, but to steer clear of them in more complex situations (e.g. I don't think I ever used one inside a procedure), in the hope that readers might see that it's in their best interests to do the same.
P.S. Limits show up in applied mathematics all the time!