SEO backlinking targeting ?

High I have been trying to research backlink sources. Having read many articles including Franks it is very intresting I must say. I have an idea which I wanted to ask about. My thoughts are Google will probably look at how close the languages and themes are of each site that link to each other. So if your subject was gardening for example a rose gardeners site might be better to link too if you yourself have a Rose garden website. However a 'random flowering plants' website still may help with my ranking if linked to with a better PR. Is anyone prepared to let me know from general experience with Google analytics what is the best guess here ? Or would it be something people wanted to keep close to there chest perhaps ?

Are Google quite loose with site categorising eg- I could seek to obtain any links from any gardening site ?

Thanks for the great read Frank with your SEO guide it was very helpful for me.

Edit : Once I reached the end I think I answered my own question.

Should people want to help I'd of course appreciate that.

Thanks very much.