Clients often come to me with a slightly apologetic air when they fear that they job they have for me isn't "interesting" or "exciting" enough. Funnily enough, while my portfolio is packed with the image-heavy layouts, I have also got a lot of experience in more copy-heavy works like books, corporate reports, etc. Perhaps it is my ever-so-slightly obsessive nature, but designing large bodies of text is a fascinating puzzle for me.
Creating a document which is legible, enjoyable, and easy to digest of many pages of dense type is certainly a huge issue. Simple things like page numbering, easy navigation, clear signposting, etc are key. Color and type weight can help, but restraint is also necessary to make something pleasurable to read, (rather than an exhausting act of will on the part of the reader, trying to ignore too many colors and typefaces).
Ideally, as with so much of the work I do, I try to design work that communicates the content above and beyond the design. That is to say, I want people to find it so easy to read that they don't think about how it looks. The look of the piece is so appropriate that it allows the content to communicate more easily than it otherwise would. This doesn't mean that there is no design, on the contrary, it can be much more difficult to design in such a manner. However, it is the most delicate touch which rewards, and so requires attention and subtle negotiation with the author to bring the work to fruition.
If I only ever designed one kind of thing, whether it was books, posters, banner ads, websites, mobile sites, comic books, CDs, or splash pages, I think I would get stale. Luckily I work with all manner of clients on an ever-increasing range of design work and each time I learn something new about myself.