You can click anywhere on the Stack photo to "drop the eyeball," and the Shelf photo of that location will load up below. The Next Stack / Prev Stack buttons below the Stack photo will move you from Stack 1 to 6.
You can mouseover the Shelf photo to zoom in, so you can read the spines. You can use the mousewheel to zoom in or out further.
Over on the left, you have a giant, scrollable list of tags and topics. All of the text in the sidebar is clickable. By default the sidebar is grouped into the Stack's "Categories," but you can sort alphabetically for the full list of Topics and Titles (Titles are in italics).
When you click open a Topic/Title, you'll get a list of other locations in the stacks where this Topic/Title appears. The Topics and Tags which appear below the shelf photos work the exact same way.
Sometimes (but not always), there will be an extra link at the bottom of this list: “Archive.org Results ➚”. Clicking this will open a new tab with search results for the Prelinger Library's Archive.org collection, where they have some scanned material on this topic.
Also: this website works like normal in desktop browsers, so you can resize the window, or use command +/- to scale the site until it looks good to you. You can also use command F to find a particular word(s) — handy for searching in the topic sidebar.
That's it! That's the whole thing. If you're looking for more information about this project (technical, esoteric, or otherwise), head over to the “About” page.