There have been some significant changes just in the last year, most notably, the movement of the xparse package into the kernel and a preference for \NewDocumentCommand et al. over \newcommand and its relatives. Most of the changes happen below the user interface level, but occasionally the bubble up beyond that (right now, there's some discussion about the fact that \NewDocumentEnvironment{foo} will silently overwrite an existing \endfoo command and how best to manage this (I'm in camp issue an error. The other possibility is to modify the default mapping of \begin{foo}…\end{foo} which currently calls commands \foo and \endfoo to do something slightly different but this would break lots of legacy code and likely won't happen). The biggest area where changes would impact the book is the appendix of error messages. In at least a few cases I've had error messages change in this year's updates (partly because of my own reports to the core dev team).
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There have been some significant changes just in the last year, most notably, the movement of the xparse package into the kernel and a preference for \NewDocumentCommand et al. over \newcommand and its relatives. Most of the changes happen below the user interface level, but occasionally the bubble up beyond that (right now, there's some discussion about the fact that \NewDocumentEnvironment{foo} will silently overwrite an existing \endfoo command and how best to manage this (I'm in camp issue an error. The other possibility is to modify the default mapping of \begin{foo}…\end{foo} which currently calls commands \foo and \endfoo to do something slightly different but this would break lots of legacy code and likely won't happen). The biggest area where changes would impact the book is the appendix of error messages. In at least a few cases I've had error messages change in this year's updates (partly because of my own reports to the core dev team).