Caliper vs. Bulk
First the easy and short one: bulk vs. caliper. When you design a print book, you may choose to specify the cover stock as 10pt. C1S. This means that when you use a micrometer, it will show the thickness of the paper to be 10 points (.138009”). Personally, I’ve usually specified either 10pt. or 12pt. stock for 6” x 9” perfect-bound books I have designed. This is a good starting point. You may want to get paper samples from your book printer before you make your decision. You may even want to choose a thicker paper stock for larger-format books.
As an addendum, this is what the C1S means: “coated one side.” For a print book cover, that means the inside front and back covers will have the same uncoated surface as the (often) uncoated paper used for the text. If your text pages are coated, however, you’d want to look for C2S (coated two side) alternatives (such as regular 65#, 80#, or 100# cover stock).
Both 10pt. and 12pt. stocks are specified in absolute measurements, unlike 60# text (used for the interior of the print book). When you choose a 60# text stock, that specification reflects the weight of 500 sheets of 25” x 38” paper. This size (which may be only one option for the sheet size a printer may buy) is called the “basic size” for that particular paper, and the 60# specification is the “basis weight.” That gives a consistent measure to all paper.
In contrast, 65# cover stock reflects the weight of 500 sheets of the thicker cover stock. The reason this makes sense is that the basic size of cover paper is smaller (20” x 26”), so compared to 500 sheets of text paper, it will be significantly thicker (even though it still weighs 65#).
This thickness (on an absolute level) is called “caliper” (as noted before, regarding 10pt. and 12pt. cover stock).
In contrast, “bulk” refers to the relative comparison of paper weight to paper thickness, and this can vary from paper to paper. And the way you can compare one sheet to another is through the “ppi” specification noted on your custom printing contract. PPI means pages per inch. One text paper might have a bulk of 350 ppi, while another may have a bulk of 400 ppi. The first has a higher bulk (fewer pages for the same one-inch measurement). You can determine the thickness of a print book text block by dividing the page count by the ppi (500 pages divided by 400 ppi would be a 1.25” text block, for instance).