I usually find "book" weights to be lighter than "regular" or "normal".

I think normal is a stupid name for a weight anyway.

IMHO there is so much variety in font styles/weights that you should go with what intuitively looks best, and the only way to figure that out is to print it on a high dpi printer. Clearly when a font has a "display" weight that is designed for larger sizes and headlines. I am frequently frustrated by the (seemingly) arbitrary way fonts are packaged, usually I want to mix and match but that's sometimes more expensive. As in, I'd rather have book and regular but maybe not light and bold.

This is also a question I wrestle with as a type designer. As far as things already on the market, there is no definitive answer. Frutiger has the best idea with his numbering system. You at least know which weight is bolder just from the number.
The terms Regular, Normal, text, book, light are sometimes used interchageably and sometimes have a clear meaning. I have also seen book look lighter than regular in some families. Patty is right, you really have to look at samples set as text blocks to be able to make up your mind. Designers who have created cuts for specific uses are giving you the most help in chosing. Many of the larger superfamilies have a cut for text, subhead, caption, and display. The cuts are optically sized to fit the intended job. Often, it is not just lightness that changes, it is usually contrast, xheight, and width as well. A caption cut will be a bit bolder, more extended, higher xheight, and have a bit less contrast than a text weight.
Another consideration is printing and paper stock. A more absorbent text paper may print a bit darker than a coated stock. Looking at a typical phone book page will give you the extreme.

I wonder what the typical Typophiler would say about text vs. book weight? Maybe it is poll time?

ChrisL

One thing to keep in mind whenever you see regular, text, book, and normal suffix type weights is that NOT ALL type families or expanded weight families were conceived or developed as a whole package.

Many revival families with book or text weights, as well as expansions to initial releases of digitized font families were added afterwards, sometimes years or decades later, often to correct or enhance design of regular weight specifically for book/magazine publishing. The necessity to have additional weights often came as responses to market requirements.

Since there are no standards associated with weight naming and actual type densitiy (not all text, book or regular weights are comparable to each other from type family to type family), you really have no way of determining what looks or works best for your projects other than testing the actual output. So follow what Patty and Chris suggest above and use your own judgement.

Thank you all for your responses. I have been thinking of buying FF Milo- which is not on the cheaply priced side of the market. Its then necessary for me, as pointed out, to generate test prints to determine the suitable package.

Mikey :-)

I wonder what the typical Typophiler would say about text vs. book weight? Maybe it is poll time?

My $.02
Regular is the roman weight of a font that just can't bring itself to be called Roman, often in a Sans that was never meant for body copy. Text is for setting long blocks of text as opposed to ad copy. Book often seems to mean that some foundry is milking a cash cow and churning out more weights of a font than are really necessary, ie. AG Buch.