Contrary to popular belief, external links to different pages on your website will not harm your home page google (or other) search engine ranking. It will, however, improve the individual ranking of other pages on your website and probably your website overall. Your home page will usually give a more general overview of your site as a whole anyway, and so may not be an appropriate starting page for many visitors – so much better to drop them at a page that is immediately relevant for them.
So, in summary, don’t automatically assume that the best place to link to from external websites is your home page. Think about where the visitor is coming from in the link and try to give them the most appropriate landing page.