The image above is from the building's Wikipedia page where it says that, "This work is from the Gottscho-Schleisner collection at the Library of Congress. According to the library, there are no known copyright restrictions on the use of this work.  Images in this collection have been placed in the public domain by the heirs of the photographers. 

The design that most resembles the finished building was arrived at in September of 1945 but the building took 14 more years to complete.  This was due to issues over planning consent and Wright's ongoing differences with Rebay and her successor James Sweeney.

In the book, The Guggenheim: Frank Lloyd Wright's Iconoclastic Masterpiece, by Francesco Dal Co, the building is described as being composed of ".... two volumes of different heights (that) rise upward, variously connected and joined to accommodate both the galleries distributed around a dilated, empty atrium illuminated from above (the “crystal court”) and the “Holy of Holies”, the space where the collection’s most important works are displayed (now called the “High Gallery”)." (see below)

However, the ramp was not popular with some of the artists whose work was exhibited in the museum.  "This revolutionary concept also had its drawbacks: there are almost no horizontal floors, with the exception of the rotunda which Wright intended as a social and gathering space, not for exhibition purposes. There are few planar walls on which paintings could be hung and the reduced ceiling height inside the exhibition ramp makes it unsuitable for displaying large artworks.".... "... Furthermore many artists felt that their works were overpowered by the architectural strength of the “container” and worried that their artworks would not receive necessary “sympathetic” attention from the visitors."  Wright of course disagreed saying that, "... No, it is not to subjugate the paintings to the building that I conceived this plan.  On the contrary, it was to make the building and the painting a beautiful symphony such as never existed in the world of Art before."

The Richard Meier & Partners Architects LLP website explains the history behind the space shown in the images below. "Frank Lloyd Wright had originally designated a small, curved room on the second floor of the Guggenheim Museum, along the windowless Fifth Avenue edge of the building underneath the spiraling rotunda, as a repository for his drawings and models of the museum. Over the years it was used as an employees lunch room and a storage area. In 1978 the museum decided to have the space converted into a reading room."  This was made possible by a grant from the Esther A. Simon Charitable Trust. Esther Simon, known as Aye Simon, was an artist, collector, and influential steadfast patron of cultural institutions

"In effecting the transformation, three original round skylights were taken as primary organizational cues. Each of the skylights focuses on a major activity area: reception, reading table, and built-in banquette. All of the furniture – including the banquette, bookshelves, tables, chairs – is of curvilinear, light-finished oak, and was especially designed for the space. .....

......  The portal to the almond-shaped room, which is entered from the second level of the spiral ramp, is in the shape of a giant keyhole or moon gate. This round shape helps to mask the transition from sloping to flat floor plane and to minimize the distortion that the canted wall surface would have accentuated in a more rectilinear form. It also echoes the shape of the elevator, which is directly opposite across the ramp. Beyond this thick-walled, ceremonial doorway, a semicircular vestibule restates Wright’s circular theme and further mediates the change in floor levels, modulating and deflecting one’s entry into the room."