My Late Mom loved Cozy Mystery novels.
I’d occasionally read some, too, but for the life of me, I cannot recall their authors or titles.
Dopers–please provide a list of novels & authors, predating 1980, of the cozy mystery type.
Not sure if they count as “cozies” or not, but I’ll strongly recommend the Asey Mayo series by Phoebe Atwood Taylor. (It has the advantage that only a half-dozen or so characters appear in more than one book, with little character development and no mention of previous cases, so the books can be read in any order.)
Based on their inclusion in the Cozy Mystery website (an amazing resource), my favorite authors from pre-1980 (at least when they started writing mysteries) are:
Dorothy Sayers
Agatha Christie
Josephine Tey
Rex Stout
Ngaio Marsh
Ruth Rendell/Barbara Vine
John Dickson Carr
I was writing a longer post, but see on preview that SpoilerVirgin beat me to recommending Christie, Sayers, and Tey. 
I’m always interested in titles & authors, the more the merrier.
That said, I got onto the library catalog, & reserved A Man Lay Dead by Ngaio Marsh
This is damn near the definition of a cozy mystery.
Now, if only the chap was offed with a mysterious poison of the East…
For many of these authors, you’re better off NOT reading their first mystery. They improved with age and practice. I love Ngaio Marsh, for example, but her first five or six are well-written but plodding. Later she gets into characters, so the stories are comedy-of-manners as well as cozies.
It sounds like she might like the short Sherlock Holmes stories…or even the longer ones, but the short ones can be banged out pretty quickly if that’s what she likes.
That’s a good point. The first Sherlock book was, um, odd.
Modern day cozies have evolved into a formula that was not as dominant in the early whodunits. They mostly look back to Christie’s Miss Marple series, set in a small town with a snooping spinster who knew everything about everybody. The Asey Mayo books are a good example of that with a male lead. The Nero Wolfe books aren’t. They’re at least halfway between the whodunit and the private eye, which became the antithesis of the cozy.
I’m not a big fan of true cozies, but I love the classic whodunits. If you try them, keep in mind that they’re not the same thing. Dex has a good point that only the very rarest of authors got it right in their first book. Worse, almost every one wrote the series into the ground before they stopped churning them out. But one advantage of the cozy style is that every book is more or less the same, so it usually doesn’t matter much where in the middle you start.
Names are in alphabetical order because I’m just lifting them from my catalog of mysteries.
Anthony Berkeley
Nicholas Blake
Anthony Boucher
Edgar Box (pseudonym for Gore Vidal!)
Edmund Crispin
Anthony Gilbert (a woman)
Francis and Richard Lockridge
Stuart Palmer
Richard Powell (the funny ones with punning titles)
Clayton Rawson
Craig Rice (another woman)
And Ellery Queen, whose series character was named Ellery Queen. Queen, as Anthony Boucher once wrote, “is the American detective story.” Order does matter for him. His (Their - Manfred B. Lee and Frederic Danney) first books were standard unguessable twisty fair-play mysteries with cardboard characters. Then he started experimenting and spent the 1940s writing each book in a different format, from women’s magazine romance to straight novelistic to the first great serial-killer thriller. But the standard whodunit was dead, killed by the private eye and noir. His later books just play with mystery tropes and become increasingly cardboard puzzle problems. Not to mention that his weirdest book, And On the Eighth Day, was ghostwritten by the eccentric genius Avram Davidson. And the almost as strange The Player on the Other Side was written by Theodore Sturgeon, after Lee had developed a crippling writer’s block. I think the best place to start with him is Calamity Town, set in the small town of Wrightsville with a feel that many cozy writers have imitated ever since.
Queen is a favorite of mine because his career is unique. No other writer of a series character ever varied the books as deeply as him, and the only one to attempt it successfully is Ed McBain with his 87th Precinct police procedurals.
“She” who? 
Your late Mom, of course. We’re all assuming that you’ll be sending her the suggestions.
Unless from her vantage point she knows all the endings.
Sorry, I kinda skimmed the OP and the responses and thought you were looking for books/stories for your mom. Change “she” to “you” and “she likes” to “you like” and I’ll stand by my post.