There’s a fine line between being helpful and giving your expertise away. And one of the places that’s evident on a website is on the links you have to OTHER websites.
Remember that when you put a link on your website, it’s a way for someone to LEAVE your site and go somewhere else. They may never come back, either.
If you want someone to hire you for your expertise on dog training, and your website is full of links to dog training tools and books and dog training forums and so on – why would someone hire you?
In this instance, your goal is to get someone to call YOU and to position YOURSELF as the expert, right? So a better idea might be to have them give you their name and email address in return for a PDF copy of that Dog Training Resource List. That way you have their contact info and can follow up with them, and you’ve subtly given the impression that you’re the “go to” source for quality information.
It might be that you get a commission for selling a dog training book – now that’s different! Making money is a perfectly valid reason to link to another product.
I’m not saying not to link to other websites – and in fact, there’s some evidence from Google that you SHOULD. Matt Cutts (head of Google’s Webspam team) has said that linking out to high quality websites is one of the many factors that they use to evaluate a site.
Just make sure that you’re thinking strategically when you do so.