Much of the online D&D community refers to Libris Mortis as "The Book of Bad Latin". This is because they are not in the intersection between the set of people who know Latin and the set of people who have read the introduction to Libris Mortis -- an intersection that seems to consist solely of me.
The community assumes that the title is supposed to translate to "The Book of the Dead" -- a not entirely unreasonable assumption. If this were the intended title, then it's true, it should be Liber Mortis, not Libris Mortis. Nominative.
But it just isn't so. Open your copy of Libris Mortis to Page 4, the very first page with actual content. Look at the bottomof the page, where it says "This book takes its name from a set of tomes penned in a dialect of Celestial [...] Roughly translated, it means 'From the Books of Dead'". This is interesting for two reasons: 1.) the title isn't in Latin, it's in (possibly bastardized) Celestial -- which is to say, Celestial is Latin, which I've taken as canon for my games -- and 2.) it's intended to mean "From the Books of the Dead", not "The Book of the Dead".
Now, if I were to try to translate "from the books of the dead", I'd say something like "ex libris mortis". But the ablative form "libris" already implies "from", so the preposition "ex" isn't, strictly speaking, necessary. Of course, "mortis" is singular genitive of "mors", so it means something more like "of the corpse" or "of death", but that's still pretty close.
So the title Libris Mortis means just about exactly what it's intended to mean. It's the Book of Perfectly Acceptable Latin. It's the Book Of At Worst Slightly Idiosyncratic Latin. Now you can look down your nose and feel more educated than anybody who utters the phrase "Book of Bad Latin".