Limit my Amazon searches to only books?

The Amazon Associates program is one of the unsung heroes of the net, actually, both from the perspective of webmasters who have earned a pretty penny through it and from Amazon itself, who makes a surprisingly large percentage of its gross revenue from Associate sales. I'm a member too.

What you're asking involves basically reverse engineering a complex form that Amazon offers in its associates area. Start by having a look at their search with dropdown menu snippet, because that offers the functionality you want, it just doesn't hide the actual category selector.

Here's how that form looks:

Of course, Amazon's doing this with an iframe, but if you dig into the source itself (try "View Source" on this page, for example), you'll find that there's a block of code that looks like:

<select name="index">
<option value="blended">All Products</option>
<option value="apparel-index">Apparel & Accessories</option>
<option value="baby">Baby</option>
<option value="beauty-index">Beauty</option>
<option value="books">Books</option>
<option value="photo">Camera & Photo</option>
etc. etc.

That's the key, the variable index. To automatically have books be preselected as the search category, and to hide that selection from the user, simply use:

<input type="hidden" name="index" value="books" />

Put that all together and here's a search box that only gives you results in the books category at Amazon.com:

<form action="http://www.amazon.com/gp/associates/link-types/searchbox.html"
method="get" name="search" target="_top">
<input type="hidden" name="tag" value="davetaylor" />
<input type="hidden" name="mode" value="books" />
Search Amazon for:
<input id="keyword" name="keyword" type="text" size="18" />
<input type="submit" value="Go!" />
</form>

In use, this minimalist form looks like:

That should get you going with your HTML task. Good luck!