There are a ton of digital resources for brand mark inspiration. But going through Pinterest boards and instagram galleries can't really compare to the feeling you get from paging through a highly curated and printed book logo modernism, the Los Lobos book series, logo, type and symbol. All pulled together massive amounts of symbols and letter forms that can help you generate ideas where you might be stuck. I can't tell you how many times I've been grateful to have these two page through there just refreshing to have when you're in the middle of a creative slump. I'd also recommend looking at art, architecture and even photography books for ideas on shapes or patterns. The book sightless, for instance, was written by an architect who wanted to push the boundaries of three dimensional form. There are 1,001 shapes here that can give you all sorts of ideas for how to create something unique. So don't forget to look for those unexpected gems.