The Beatles remain the most documented band in history.
If you want to know what it was like in the studio, there are plenty of resources available to you. The Anthology series, both the film and CD, is a must have, for example.
But if you want to go deep, you have to hit the bootlegs. Hahst Az Son is essentially four sides of the tape running during the Get Back recording sessions (or the making of the Let It Be film, if you prefer), and it’s every bit as cool or boring as you think it is. Want to hear false starts, or Paul calling out chord changes? You might not, but I do.
The best chunks on this boot are available legally and in much higher quality on Anthology, but if you’re a completist you want this. The asking price on this one tends to hang around 45 bucks, but 20-25 is more realistic if you pick it up on an auction site or from a good record store.
As for that weird title? It’s probably a reference to Hot As Sun, a mythical lost Beatles album that was mentioned in a 1970 Rolling Stone article but never surfaced.