This is a luscious album starting from Bobby Haggarts's ''What's New'' to Gordon Jenkins's ''Goodbye.'' Miss Ronstadt takes a jump - with thrilling confidence - from country-rock to popular big-band music - some going back as far as the '20 s. (She has already tackled operetta in her stage, film, and recorded versions of ''The Pirates of Penzance.'') Her version of ''I've Got a Crush On You'' is my own favorite here. Her creamy voice envelops the role. It will be interesting to see how much of a success - in record sales, that is - that this LP becomes. Ronstadt's fans are accustomed to hear her belt out rockers. (The New York Times reported recently that in fact sales of this album were going hearteningly well.) The oldies here have a welcome kind of innocence to them, especially compared with the battleground a Pat Benatar screecher can have. It's important too to pay tribute to the musicians here - the Nelson Riddle Orchestra. Riddle himself did the arrangements here. He's a natural for this. His collaboration with Frank Sinatra had brought several of these songs to fame 25 years ago. For my taste, the arrangements seem almost too flowery for a voice with the strength of Ronstadt's - but that comes from one who grew up on rock. The orchestra's contribution is usually quite subtle; Riddle stands back and lets Ronstadt do the work. The orchestra puts a sheen on her performance, and its beauty is really in its, for the most part, non-intrusiveness, leaving the main act to Ronstadt's pearly pipes. But how could this ever catch on with today's jaded rock buff? And yet, if it did . . .