Looking for good recordings of female vocals I have not yet tried...

Eva Cassidy, Songbird, Time after time.

Linda Ronstadt (no doubt mentioned earlier)

Not sure if these have too much background:

Dancehall Crashers (Honey, I'm Homely) and Suspect Bill (Bill Me Later) - neo swing.  I use a couple of these to test audio

Courtney Love (if you can ignore the baggage)

annie ross "A Singer, a Swinger, A Gasser"

The quintessential female jazz singer is Peggy Lee. Check out the album The Ultimate Peggy Lee, it is recorded very well and her style is so unique and subtle while extremely effective. Songs she made popular, 'Why dont you do right' with Benny Goodman, 'Fever' in 1958. She wrote and produced , 'I don't know enough about you'.  Many more including the classic, 'Is that all there is'. Just a very under rated songstress from thr 40s and 50s. Many say she was the female version of Frank and I agree. Adding morr fuel to the fire they are reportedly to have hooked up frequently. Not surprising.

Cassandra Wilson and Norah Jones, both on Blue Note Records (and in audiophile pressings by Analogue Productions).

+1 (I'm partial to NJ's "Come Away With Me" and "The Fall").

+1 for Alison Krauss/US "Live".  Incredible recording and her voice is heaven-sent.

I might also suggest Patricia Barber.  Her recordings are well-engineered and she has one of those "smokey" jazz voices.  Not to mention some very wellp-written pieces.