Use a companion font for the heads & subheads

This font family needs to be carefully chosen. It needs to have enough contrast to the body copy font to make the heads stand out. But it must have the same, or a very similar, x-height. This way you can use it comfortably for things like run-in heads, where the first few words in a paragraph are used as a contrasting low-level subhead. Your family choice should probably have a similar width. It should emphasize your stylistic decisions which were made to appeal to your readership. These choices are the main determinant for the basic historical and decorative style which is used in the niche with whom you are trying to speak.

There are very few serif/sans combos which work for run-ins

The difficulty is in finding similar x-heights and pleasing weight combinations. You can do very nice things when this is used. In the following paragraphs, notice in the first example using Buddy/Contenu, that a regular sans gives a beautiful, subtle contrast to the text font used for the rest of the paragraph. Click to enlarge.

The Futura is much too large and heavy for the Garamond. The Verdana/Cochin combo is very bad.  The Gill Sans/Jenson combo is pretty good.

Now look at now different it looks with a bold version of the sans.

With the bold sans fonts, only the Buddy/Contenu pair works at all. In the other three, the sans is much too strong and the x-height differences have become irritating. None of the bottom three pairs could be used for run-ins. Click to enlarge.

You really need to think of these things as you begin to design and lay out your books. For novels it doesn’t matter as much. But for non-fiction it really makes a difference.