availability of public records online

Some public records are available, and some aren't. The records are usually available from where ever they are held, but they may also be available from a central source. The easiest source for death records in the U.S.A. is the Social Security Administration.

You can actually learn a whole lot of this stuff for free. SSDI, available free (just google it) on several websites can confirm most deaths. Many states have court records available for free at their judicial branch websites, you can get some marriage, divorce, birth, and address records on the free portions of genealogy.com or rootsweb. PIPL Search will turn up some records for free. A trail membership (about $10.00, I think) will turn up more specific info. Some states have free databases of inmates. You just have to look around, it takes time.

AND, if you know where to ask, and your story is convincing, folks that have access to paid databases and/or records offices will sometimes volunteer to take a look for you.

I was a searcher for years. I found people with only a first name and just a little info about an event that had taken place 20, 30, 40 years in the past. Almost anyone can be found for almost no money. It's just work and time.

The other posters mentioned the social security death index.
Another is Property owners records online
If they own property anywhere, their address will pop in this database.

Depending on how far back you're looking, some states have free death databases for older records, like California (http://vitals.rootsweb.ancestry.com/ca/death/search.cgi), West Virginia (http://www.wvculture.org/vrr/va_select.aspx),
Ohio (http://ohsweb.ohiohistory.org/death//), and Illinois (http://www.cyberdriveillinois.com/departments/archives/databases.html). Many of these are available on their own websites, others thru the new Mormon pilot project. (http://pilot.familysearch.org/recordsearch/start.html#start)

Other items (for example, older Michigan marriages) may possibly be found through the pilot (note: this has only come online the last year or so, so depending on when you're setting your story year-wise, that could present an issue). It's a goldmine for this stuff, and sometimes you can view the actual records online.

Some states are way more draconian about having access to their records. So it depends on where you want to set your book. Alabama, Iowa, and Idaho are three of the states that make me crazy in trying to access records if you're not immediate family. If you are, it's a bit easier.

There's also a wonderful online service called Random Acts of Genealogical Kindness. (http://www.raogk.com/listing.htm)

People from various areas volunteer to go and get information for other people. Some will go do courthouse searches, others get graveyard photos or the records from the cemetery, or obituaries, etc.

I've used this about a bazillion times to write my family history since I have folks spread out all over every state in the union and four Canadian provinces.

You can often get a free 1-month trial to the for-pay databases, such as Ancestry.com

And, if the person has an uncommmon name, systematic googling for the name and tracing the mentions.

The data may be online in some states/counties and not others.