This is a great article by SEO Moz which explains the practice of interlinking (and link building) between sites. Why is this important? Because if you are hiring an SEO firm that is promising you fast results by building out hundreds of sites that interlink in order to get you ranked, it could get you in trouble in the long-term. Find out when it is, and isn’t, okay to interlink betweeen sites by watching this great tutorial from one of the “white hat” experts in the field.
Their key points:
1. Do it for humans, not for search engines.
2. Stay away from spammy link building. The search engines will figure it out, and your site can get “punished” as a spam-heavy site for it.
3. Be authentic, because Google is THAT smart, and will figure it out. Black hat tactics will get you punished sooner or later.
So in the end, interlinking your own sites is okay, IF it makes sense for your readers.