My two pennies; my first education was an apprenticeship in bookbinding... (book-geek, yes)
The classical way to define frames for content on book pages are based on the 5-7; and allow me to point you to CraigMod - he knows what he is talking about:
Image of page layouts
Craigmod - books in the age of the ipad
The construction of these things take into account a visual corrections; just as with passepartous for pictures.
Determening the printing area for books have been going on for centuries. There are two main, classical layouts:
Van den Graaf, who took "reverse engineered" old books came up with the 2:3 ratio:
Another tradition it to base it on Tschichold approximation of the golden section, a ratio of 5:8:
Personally; though Scott gave a good answer, I would skip the creep (of pages! not saying Scott is a creep! ;-D). What is a very important element here, though, is how many pages you imagine the book to be, the type of binding and the weight of the paper.
I would never have equal left-and-right and top-bottom margins. The text-rich layout will give a feeling of text "sliding" or falling off the page. Consider the passpartous on well-framed pictures: they are equal left-right, but this is because there is no spine, no paper-bending/folding. The proportions of a passpartou is to have a larger margin on bottom than on the top. This will sometimes vary depending on the picture itself (darkness, colour of frame, passpartou etc), but the bottom should always be a little higher than the top. Some traditionalist say something like the combined width of left-right for the bottom. Personally; I adjust a little depending on the nature of the picture.
As when it comes to fonts. This is highly subjective. Since your book will be text-rich/dense, I would go with a serif for the main content. There are many lovely and useful serifs out there; you are not stuck with Times New Roman. Personally; I would suggest take a look at Didot, Garamond, Bodoni, Hoefler Text, Palatino.
If you insist on a sans-serif make absolutely sure you use one that is well constructed. There are a lot of rubbish sans serif fonts out there. Guessing you might need æ, ø, å, pay particular attention to these characters. Excellent fonts of either type will actually have a slight variation on x-heights to compensate for a sense of heavyness. This example is Optima by Herman Zapf: notice the o going slllliiiightly lower, and part of the r slightly higher than its own stem. This is a sans serif with a softer feel; a hint of serif. It is one that actually works on longish texts.
For headings, quotes and other font elements: either use the same font, or go to the opposite. Be very, very careful about "arty" fonts. As someone said; you do not need a font that looks like dogs, to make the content bark.
Fonts will largely also depend on what kind of feel the book should have. Contemporary? Historic-ish (and if so, from where?) Arty? Avant-garde-ish?
I might be a little too font-geeky, but I found a book once, a story from London, set in an italian font some few hundred years later. I had to make an effort not to let that distract me :-D
Again; this depends on the font and content. And of course if you have a limit on number of pages. If you decide on a font and a font weight first, you must experiment with line spacing. It is entirely dependent on the font you choose. And... also dependent on wether you use a right-ragged or not.
Sorry if this was overwhelming.