PHP and Web Services (where to start?)

Hi all, I'm a total newbie to programming, but I picked up PHP last month and redesigned my website (I think it's great). What I want to do now, is create a price comparison engine of online bookstores using PHP and whatever web services the vendors supply. My question is: where should I start? I've purchased the Professional PHP4 XML book by Wrox (being shipped), but am wondering if anyone has coded something along these lines. Sorry for such a basic post, but I feel lost and don't know who to ask. Thanks in advance! Stacy stacy@christianbookpreviews www.christianbookpreviews.com