Oh dear. Sorry for messing that up. I have four Random House large print books right now and none of them say what the font is. They have very small serifs, as far as I can tell.
Repeat after me.
There are no rules in book formatting. Only conventions that can be tossed aside when they aren't working.
Once more...
There are no rules in book formatting.
When it comes to identifying fonts, it's hard. I can identify a handful of fonts by sight, but keep in mind that it's because this has turned into an obsession for me. Even some of the most respected typographers can spend hours identifying a font. Adobe Garamond Pro happens to be used for a lot of books, so it's one of the easier ones to identify. The US printings of Harry Potter use Adobe Garamond Pro. And even if I am sure it's AGP, I'll still grab a a picture of it and run it through identifont just to double check.
I don't think it's necessarily wrong to us a sans in a large print book, it's more that it's outside of the current conventions. And considering you want to save space... using a serif will help. Also, it's okay to set the text as justified and not flush left/ragged right. (This will also save some pages). I then adjust the letter spacing and word spacing so it doesn't have some lines more spread out than others (or more constricted).
I formatted a Large Print book for a friend and set up a template for her to use based on some of the best practices I pulled from several large print books I pulled off the shelves of different libraries.
To start...
Trim size: 6x9 (and we've already discussed nudging that up a little)
Margins: Top 1"; Bottom 0.9"; Inside 0.85"; Outside 0.8" (That inside margin can grow to accommodate page count and at the same time decrease that outside margin. The outside margin is very generous, so there is room to adjust without pushing a too tight outside margin. I would probably say, 0.65" is the smallest you want to aim for, but 0.60" isn't too tight for large print.)
Font: Adobe Garamond Pro (I recommend using an Old Style serif if you don't want to use/like AGP.
Size: 16 pt
Leading: 18 pt
Kerning: Metrics
Hyphenation: At least 6 letters, after first 3 letters and before last 3 letters, limit to 1 hyphen. Basically, I allow for hyphens, but have no problem going through and removing a hyphen if I think there are too many.
Justification (Spacing of words and letters):
Word spacing: min - 90%, desired - 100%, Max 120% (I'd rather have more spacing between words than less)
Letter spacing: Min - 0%, Desired - 0%, Max 3% (again, I'd rather more spacing than less)
Glyph Scaling: 100% across the board. (I will play with these for non large print books and some fonts, but not large print because large print is about creating uniform letter shapes and scaling in any direction goes against that concept.)
If you take these settings, your formatter can make adjustments to get a similar look and feel with any other typeface. (I happen to love Sabon, also a Gramond, and feel as though with some adjustments, it would work for a large print book.) Nothing says you have to use these exact settings, but they are a good jumping off point and can be adjusted to fit your needs, such as keeping your page count down.