This is a question I've thought about for years, having just published my twentieth book, and I've concluded that there are three basic approaches you can take:

First, you can have a Web site address or password code that's printed in the book. This would work until your readers shared it with others, but it would serve the purpose of preventing random strangers from wandering onto your Web site and obtaining access to the information therein.

Secondly, you could actually do what old-time computer games used to do to verify your ownership of the original content: randomly prompt for "word X on page Y" out of the book (games used to ask for a specific word out of the instruction manual). Again, this would serve your ostensible purpose, but I fear that the sheer annoyance factor of having to constantly reference the book to get onto the site would prevent anyone from ever visiting twice.

The third solution is to just open your site up, as I finally decided to do with my Web site for Wicked Cool Shell Scripts. It looses the aura of exclusivity, but it gains visibility and helps with overall book sales. I know that I have sold many copies of this title because people get to the site, check out what's there and decide that for $20, it's well worth buying a bound copy of the entire manuscript from Amazon or another bookstore.

This doesn't address the cross-marketing concern of your partners, but I would encourage you to really question them on that. Here's what I've learned from the retail trade: for every person who tries to defraud a company out of a promotion, there are ten people - or more - who find it sufficiently enticing that they'll make a purchase decision they wouldn't have otherwise made. In this case, what I'm saying is that even if some percentage "X" of people who went to your partner's sites through your promotional links weren't actually book owners, I bet that the value and increased business from legitimate readers will far outweigh the ostensible cost (and is it really any sort of cost when they become customers while they otherwise wouldn't have purchased the product or service?)

I've talked about this before, actually, and counseled clients away from what I believe is the myopia of being so worried about pre-qualifing potential customers for promotions that they minimize the effectiveness of the promotion in the first place.

Give it a shot and let us know how it goes. And definitely let us know about your publication schedule with your new LinkedIn book too!