Apply this stylesheet to the introduction chapter, or embed it in the header. It tells a standard paragraph to have no indent, but a paragraph following another paragraph to take one. Using em rather than an absolute value allows for reader-changed text size.
Note the spaces before and after +
To keep the whole book using the same stylesheet, replace p with .p1 (or whatever) and use <p class="p1"> in that chapter. More work, though.
p{
text-indent:0;
}
p + p {
text-indent:1.8em;
}