From the table of contents I can see that there are some use-case oriented chapters in the book (like for example 12th: Real-world applications of clustering), so I'm interested if they cover use of Lucene/Solr along with Mahout or rather explore some more isolated applications?
We did not talk much about the integration of Solr and Lucene with Mahout. You are correct that this is a good form of data for processing with Mahout, but I think that in actual production cases, it isn't a really large part of the mix. Perhaps it should be, and perhaps more explanation would help that.
On the other hand, another forthcoming Manning book, "Taming Text" definitely does talk quite a bit about this integration.