Linda Ronstadt: ''Get Closer.'' (Asylum 60185.) It's really too bad that radio listeners have only heard ''Get Closer'' from Linda Ronstadt's album by the same name. It's other treasurers are abundant. With the exception of the last two songs, which were recorded several years ago, the singing is powerful and shows the growth of her vocal confidence since her service with ''The Pirates of Penzance'' on Broadway. She works with a diverse selection of other people's songs, but it all blends smoothly together almost until the end of the second side. Her artistry is showcased early on ''The Moon is Harsh Mistress'' the second cut. Kate McGarrigle's ''Talk to me of Mendocino'' reminds one of ''Long, Long Time'' from her early years, except that there's been amajor style change -- instead of a country-style wailing, the delivery is clean and in this particular case, almost detatched. ''I Thing it's Gonna Work Out Fine,'' which she sings with James Taylor (in fine voice himself), makes you hope they will do more singing together. However, the last two cuts don't belong on this album. Stylistically, these songs would perhaps work in better with some of her earlier work -- in fact, they are some of her earlier work.