I just learned some things.
I wanted to add some commentary on one of the most memorable features of "Houses of the Holy"-- its outer and inner cover art, an overly saturated painting of a half dozen nude children climbing up a strange basalt rock structure. The inner image shows the same terrain, with a child being lifted towards the sun. Given Led Zeppelin's interest in the occult, and the sexual nature of much of their lyrics, this album cover hovered on the border of tasteful, even joyous nudity, and something just a little more nefarious.
Apparently, I missed something about this cover: it was actually not a painting, but a photograph, or, apparently, a painted photograph. It was designed by Hipgnosis, the British design firm that designed much 70s album cover art, including most famously, Pink Floyd, The Dark Side of the Moon. The album artwork was loosely inspired by an Arthur C Clark book, and was photographed at the Giant's Causeway in Ireland over two days, with two models, with the final cover image being a collage that was later processed. It is a striking and memorable image, for me clearly Led Zeppelin's best album cover.
I do have to say that like much of what Led Zeppelin did, the album cover does strike a note for me: on one hand it seems pastoral, innocent and mystical, but I also feel (perhaps unjustly) just a little bit of unease about the presentation of nude children by a band with a history of sexual lyrics.
Also, most of what I know about the origin and making of this album art comes from a few articles, so there may be more to the story: