My introduction to letterpress printing came in August 2007 when I started working at Paper Source. We sell letterpressed invitations as well as boxed sets of letterpressed cards. At first I was like "ho-hum." And eventually I was like, "That is fantastic." Back in the day, letterpress printing was the most common form of printing. Newspapers were letterpressed. Books were letterpressed. Everything was letterpressed. Now, it is usually just found on higher-end greeting cards and wedding invitations (and prints and stationery and baby announcements and business cards). It is considered more of an "art" and something special as opposed to an everyday printing process.
The drawers are divided into what is called California Job Case style and the letters are housed in separate boxes. (The site where the job case image is from called it the QWERTY of the post-Gutenberg era.) Believe it or not, I am getting good at remembering where each letter is stored.
When you are typesetting a block of text, the letters are placed in the reverse of how they will be read on the page. Metal pieces called "slugs" divide the lines of text. Hopefully seeing it like this explains why it is not worth it to run a single print of anything. Too much work goes into setting up the text. You can see above how I accidentally placed a letter "u" as an "n". "b" and "d" are also easy to mix-up. And the expression, "mind your 'p's' and 'q's'"? That comes from the letterpress days.
The text block is carefully transferred from the tray to the bed of the press and then packed tightly with wood pieces called furniture. A chase pushes up from the bottom and an expandable metal piece called a quoin is turned with a quoin key to put pressure on the block from the side. Little pieces of copper and brass get wedged into the text to eliminate remaining loose space. Everything must be secure and "locked-in" to guarantee a clean image when the paper is rolled on top.