I really wish we'd switch to the ISO-216 sizes. A4 to A3 is so much more enjoyable and painless than Letter to Tabloid. This kind of thing came up often while making posters for campus because it was cheaper to NOT print out all of the posters on Tabloid. Rather, using a mix of Tabloid (for the important poster posting spots) and Letter (for unimportant spots and as flyers for handout). Always ended up having to spend more time to make two layouts because of it. And A4 to A5 for booklets without having to re-layout the document is a huge plus too! And for books, I like to use B4* to fold in half. (B5 pages are nicer than A5 pages for the kind of books I tend to read.) The proportions are all the same, so it just works! One layout, many paper size (of the same proportion) possibilities!

Edit: Additional note with regard to the book publication (depending on printing and binding method used)... Sometimes/often this is printed on something like A3 size for bleeding and with crop marks. The A3 paper is then folded and cut, so that the pages become B5 size. (Signatures essentially composed of folded B4 as a consequence of the cropping. There is of course difference between the outermost sheet and the innermost sheet in each paper signature, which is why it's all cropped at the end.)