A considerable amount of work has been done on Jules Verne's Mathias Sandorf by the Istrianet.org website workers. Go to http://istrianet.org/istria/literature/index.htm
Once there, navigate to: novels and novelets and click on the Mathias Sandorf link.
There is a lot of excellent information on this page.
Once there, for a superb synopsis, click on Synopsis.
Mr. Davor Sisovic from Pazin, Croatia also maintains an extensive Mathias Sandorf website.
Go to: http://www.ice.hr/davors/Sandorf.htm
Ed Brumgnach at Queensborough Community College http://www.qcc.cuny.edu/ecet/Faculty/brumgnach.asp
There are hundreds of web sites on Jules Verne.
Some of these offer books that are now in the public domain that can be read on-line.
Here are some links. Have fun!
Jules Verne http://www.unmuseum.org/verne.htm
North American Jules Verne Society http://www.najvs.org/
Jules Verne Virtual Library http://jv.gilead.org.il/works.html
Free public domain e-books http://www.jules-verne.co.uk/
The secret adventures of Jules Verne Fan Network http://www.sajv.net/
The French Portal on Jules Verne http://www.fredericviron.com/verne/modules/news/
Wikipedia on Jules Verne http://en.wikipedia.org/wiki/Jules_Verne
The Jules Verne free on-line library http://verne.thefreelibrary.com/
Andrew Nash’s Jules Verne website http://www.julesverne.ca/jvlinks.html
The complete Jules Verne biography http://jv.gilead.org.il/biblio/
Electronic books on-line http://www.classicbookshelf.com/library/jules_verne/
Jules Verne books on-line http://www.literature.org/authors/verne-jules/