I have a number of books that I like to keep nearby when writing, including a dictionary and a writer's thesaurus. When it comes to checking spelling, my preference is to use a very small book that I've owned since college titled "Follett Vest-Pocket 50,000 Words". It is small enough that you could easily fit it in a portable typewriter case for example. I imagine that it would fit in most ultraportable cases as well.
That said, I do sometimes use my smartphone or laptop on those occasions when I'm typing upstairs and left my reference books downstairs in my office.