For your marketing library, DM News publishes guides on various topics like Search Engine Marketing and Direct Response Television that serve as good overviews of each topic.
And don't forget you can go right to their website and get a print subscription to DM News for free (just tell them you're a copywriter). Been subscribing to DMN for a while now. I couldn't believe it when I found out copywriters can get it for free...unless they've changed that policy.
Sadly the freebie is only for US subscribers! I do read Target marketing though, and find that useful -- especially now that the issues are current, rather than being about 3 months behind! Useful to keep an eye out on what happens outside my market, and what I can borrow as ideas.