Hmm. It ain't ever quite that simple. If the sizes used are similar, you'll have to take a close look, as to weight, as to compatibility. If the sizes are quite different, almost anything reasonable works.

I do think if I were using an OpenSource font like Gentium & wanted a sans, I'd pay close attention to Lato

http://www.latofonts.com/

Which in it's own right is an excellent font family. That it's free & open source is just gravy.

BTW, when we've used Gentium, we've found the Book -- available only in basic-- has better weight for text for our uses, which includes both print and the smaller size tablets, such as the Kindle Fire & Android phones.

Gentium of itself is very good for a free/libre font, but the ideal font choice always depends on context:
What is it about?
Who is it for?
How much text?
What kinds of illustrations?

hhp

Thanks for the comments!
It is a philosophy book, about 250 pages with much technical stuff like formulas, arguments, blocquotes etc. and many (really: 6!) levels of headings; which is why I need some means to distinguish them without getting too messy and Myriad offers that (and it is not so cold and sterile as most sans-serifs, which is why it seems to me that it might work well with Gentium). No illustrations, just 2 diagrams; intended for philosophy of religion nerds. The choice of Gentium is determined by the house style of the publisher. Yes, I am using the "Book" version.

I think it's OK, but I would test Gentium/Source Sans and Gentium/Tisa Sans combinations.

A possible issue is the need of special characters in headings, so check if the sans does offer them.

and Myriad offers that (and it is not so cold and sterile as most sans-serifs, which is why it seems to me that it might work well with Gentium...

Or perhaps a more cold, sterile look would be exactly right. If this is academic Anglo-American philosophy, often the authority of a "sterile" sans would be more in keeping with the discipline than a humanistic one. Metaphysics has been banished, don't you know (not to mention all that soft, soapy stuff...).

Knowing the subject matter, you might take a look at Thomas Phinney's Hypatia Sans. Yes, I know there is a humanistic element in Hypatia, but the geometric element might go well with the topic, and I think will work nicely with Gentium.

http://www.adobe.com/type/browser/landing/hypatia/hypatia.html

Edit:

As I remember, there is no bold for Gentium. Depending, you may feel this lack if there are many formal propositions in the text... Not too many proofs use bold, but occasionally it is needed. If the publisher want's to stick to Open Source, Charis may suit the text better -- though I'm not sure it has all the needed logical/mathematical operators. The publisher needs to either inform you of the requirements of the text, or pay you to go through it & discover them. Good luck with it,

Gentium (Plus) does not have any bolds, true, but both cuts of Gentium Basic do, both bold and bold italic.

Thanks, Charles, I will consider Hypatia Sans, I quite like it. But as you mention it: Metaphysics has been banished some 90 years ago, and a lot has happened since then :-) This book in fact places itself in the nowadays already thriving metaphysics-revival strand. If there is a kind of impression I would like to avoid, it is the sterility of Carnapian and Quinean anti-metaphysicalism.

This book in fact places itself in the nowadays already thriving metaphysics-revival strand.

Metaphysics never went away -- it just migrated over to the English department, under the guise of deconstructivism, etc. Are you saying it is now moving back to academic philosophy?

& a reminder, the usual caution in designing is to select fonts that will make the reader comfortable. Nothing like an old, familiar pair of shoes to make you pull out the wallet & buy the book (to mix a metaphor). If you want to "challenge" a reader (an oft-heard remark by people who teach design, but do not hire out), write your own damn book. While the reader (buyer & hopefully enjoyer) is usually the most important member of the audience, if something is written by a famous author, or an author the publisher wants more work from, by all means consider their preferences. Finally, remember the title page is for you (as designer), the author, and esp. the author's mother. Have fun, no one else will comment on it...

write your own damn book.

Good advice, but a designer who can't can still speak through Design.

hhp

Sorry for the drop-out, too busy...

Thanks for your comments. I would never call that thing in the English departments a "Metaphysics". But yes: there is a metaphysics - and I mean real metaphysics, of the Plato-Aristotle-Aquinas style - revival in philosophy ever stronger since the 60's. But back to typography: I have no ambition to "challenge" anyone through type; I just want the typography to be good: as good as possible with my limited means. For my profession (I mean the one I am paid for) too is not typesetting but writing philosophy books :-)

I have settled on keeping the Myriad as my sans-serif font; here is a specimen of the result:

Thanks everybody for your feedback!

the all-capitals lines need wider spacing.