Continuing to work on my AllegroGraph book
I don't think that the market will be large for an AllegroGraph (AG) book, but after using AG on one customer project and experimenting (off and on) with it for several years, I decided that it was Semantic Web technology worth mastering. AG is a commercial product, but a free server version (supports Lisp, Ruby, Java, and Python clients) is available that is limited to 50 million RDF triples (a large limit, so many projects can simply use the free version).
AG supports the Sesame (an open source Java RDF data store) REST style APIs so if you stick with SPARQL and only RDFS reasoning, you get portability to also use a BSD licensed alternative. That said, my reason for using AG is all of the proprietary extra goodies!
In addition to a few Lisp, Python, Ruby, and Java client examples, I am going to incorporate a lot of useful Common Lisp utilities for information processing that I have been working on for many years: this will motivate me to package up a great deal of my Common Lisp code and release it with an open source license. I plan on releasing the book for free as a PDF file and as a physical book for people who want to purchase it. The book and the open source examples should be available before the end of this year.