I  discovered an interesting index in the ESTC called "genre" (search term
  GNR).  After a little detective work, I determined that this index is an
  alpha variable, so I moved through the alphabet from A to Z, searching
  FIN GNR A, etc.  I then scanned the results to wager a guess as to what the
  genre actually was.  There may be multiple letter combinations, and I leave
  these to other, more disciplined detectives.  In my single-character
  searching, I found the following genres:<p>

     A    Advertisement<p>
     D    List, or guide<P>
     P    Proposals, prospecti<P>
     S    Songs, sonnets, poems, plays (?)<P>
     V    Almanacs, astronomy<P>

  With these in my tool-belt, I set out to chart a very basic course of
  advertisements (my genre of choice) in the period 1670-1809.  I first 
did a count of all titles in the ESTC by decade, using the wild-care "#" 
(i.e. FIN IYR 167#).  I then qualified each search with the phrase "AND GNR A"
 to arrive at a percentage.  The results are presented in the following 
table (I will print a graph to bring to class on Tuesday).  It is much more 
dramatic than these data are.)
<pre>
   PERCENT OF ADVERTISEMENTS IN THE ESTC, 1670-1809  As percent of total titles
       (the label indicates that decade, starting with the labeled date and
              ending with the last year of that decade, e.g. 1670-1679)

  1670:   .1%
  1680:   .7%
  1690:   1.8%
  1700:   1.2%
  1710:   1.2%
  1720:   1.8%
  1730:   1.5%
  1740:   1.3%
  1750:   1.5%
  1760:   2.1%
  1770:   3.7%
  1780:   4.2%
  1790:   4.2%
  1800:   3.5%  (incomplete period)
</pre>
  What I found was an interesting trend in the growth of advertisements 
during the 18th century.  I am unsure as to the particulars of whether these 
ads appeared entirely separate from other titles, or within journals, etc.  
At any rate, as they appear in the ESTC, they show a steady percentage during 
the first 100 years of my search period, hovering around 1.5%.  Then the 
noteworthy phenomenon occurs: the number of ads rises dramatically around 
1770, reaching nearly 4% from only around 2.1% the year before.  This increase 
holds throughout the end of the ESTC's possible search range, around 1809.<p>

  From my undergraduate English days at Kenyon I know these findings are
  associated with a growth in consumer commodity in England and the US,
  particularly considerable demand in England for products from abroad.  Of
  course, many other social, political and economic factors can be listed.