You'll notice every review here is five-star, suggesting that only die-hard fans bother coming here to share their enthusiasm. You'll also notice there aren't many, suggesting a very special interest. Both assumptions would be wrong.
Richard Brautigan is still best-known for his 1960s novels; the poetry books (though intriguing) were more uneven, less rewarding. So where does that leave a mid-70s poetry compilation?
As it happens, Richard wasn't running downhill; he was running away from the simplicity that dominated most of his 1960s work. This change diluted his novels -- he never really was a novelist -- but his poetry improved dramatically. But it was too late: in 1975, people weren't interested anymore. And so this has become the hard-to-find-and-never-reprinted gem it is today.
Read ROMMEL DRIVES ON INTO EGYPT, or THE PILL VS THE SPRINGHILL MINING DISASTER, and you appreciate Brautigan's perspective, but only about 50% of the results really justify themselves. MERCURY finds him more mature, less playful -- but still finding his mark.
He had always been wistful, often quickly turning away just as the real poignancy surfaced. In MERCURY, he does not turn away.
Hence the five-star review. For comparison, only CONFEDERATE GENERAL and TROUT FISHING merit equal rating. 4 stars for IN WATERMELON SUGAR. 3.5 for REVENGE OF THE LAWN [short stories] and ROMMEL and PILL, and THE ABORTION. 3 stars for the 1970s novels [WILLARD AND HIS BOWLING TROPHIES; THE HAWKLINE MONSTER], and 2.5 for the 1980s novels [TOKYO-MONTANA EXPRESS and SO THE WIND WON'T BLOW IT ALL AWAY].