Even Philip Johnson had to admit it: The lobby of his Boston Public Library building didn’t turn out quite right.

The renowned ­architect, who designed the 1972 addition­ to the library’s Central Branch in Copley Square, confessed his regrets in a 1994 book, writing that last-minute changes by others had undermined his original vision of the library as a grand civic space.

“We never did get the entrance straight. The requirements for security changed exactly at that moment, and I had none of those details as part of my design,’’ Johnson wrote. “Another company did it. I kept fighting, but I kept losing.’’

Now, a $78 million, three-year renovation project, intended to restore some of Johnson’s original vision for the building, is nearing completion.

The construction, overseen by Consigli Construction Co. and set to end this summer, has been tricky at times. The library needed to remain open throughout the project, necessitating off-hours work and noise controls to avoid disturbing patrons. A new connection between the Johnson building and the 1890s-era McKim Building required what one engineer called “structural gymnastics,’’ including supporting a fragile, century-old wall with underground hydraulic jacks and propping up the ceiling with a bramble of steel shoring.

But the end result, library administrators and project architects say, will be a modernized library that’s far more inviting and useful to the public.

“We have to evolve and become a 21st-century library,’’ said Eamon Shelton, the library’s head of major projects. “This was an inward-facing fortress, and now it’s going to be the total opposite.’’

Even with intensive construction still underway, a recent behind-the-scenes tour made clear the renewed promise of the building, which is being improved as much by subtraction as by addition.

Gone are the foreboding black windows and the bizarre granite “planks’’ that acted like a barricade around much of the perimeter of the Johnson building. They were an ill-conceived compromise between what Johnson wanted — no windows at all — and the library’s trustees at the time, who insisted on at least some natural light.

“We’ve been living with that for 50 years,’’ sighed Cliff Gayley, a principal and architect at William Rawn Associates, Architects Inc., which designed the renovation project. “The windows were tinted like the sunglasses of a state trooper.’’

The psychological effect of this de-fortification is dramatic: By bringing the sidewalk flush with a long facade of clear, two-story windows, the architects have made the building — and even the institution it houses — seem infinitely more approachable. If any Bostonians still associate the library with dusty archives and shushing librarians, that image should be easily dismissed once they approach the renovated building from Boylston Street.

Through the glass, passersby will see a cafe run by the Catered Affair, a bookstore-like selection of “new and novel’’ fiction on tables, a state-of-the-art television studio for WGBH with a retractable glass curtain, and interactive screens where patrons can peruse the library’s extensive digital archives. A loose, friendly vibe is apparent even in the unfinished space.

“That whole block of Boylston has been a dead space for years,’’ said William Rawn, who along with Gayley was a principal architect on the project. “A library is the most public building we have in our civic culture. To have something like that be so kept away from the citizenry seemed to us a really sad state of affairs.’’

The new design also removed interior walls that for decades boxed in substantial sections of the ground floor, revealing the full sweep of the main hall and creating a continuous, light-filled open space along Boylston Street.

This change is in line with Johnson’s theory of “procession’’ — essentially, the series of experiences a person has walking to, into, and through a building. Johnson believed a visitor should experience “the shock of big space’’ upon entering a building, a moment that the new incarnation of his Boston Public Library addition will finally deliver.

“We tried to bring back the ideas Johnson started with,’’ Rawn said. “We were trying to figure out how to achieve the library’s goals of improving its services and opening up to the city in a meaningful way without running roughshod over the building’s history.’’

Another major “procession’’ improvement is an enlarged connection between the Johnson building and the adjacent McKim Building, which had previously been linked by narrow, disjointed corridors that confused visitors.

“Anytime we were in the Johnson building we’d be approached by people saying ‘can you tell us where the McKim Building is?’ ’’ Rawn said.

Other upgrades are less apparent, but equally important.

In the basement below the lobby, there’s a bigger, state-of-the-art facility for members of the library’s digitization team, who will be able to scan and upload photographs, books, and other holdings more quickly.

Access for people with disabilities has been improved.

The mezzanine will have new computer labs that can host classes.

And a revitalized Kirstein Business Library and Innovation Center will ­offer job-seekers and entrepreneurs the latest in production and design hardware and software.

There are still nooks for lone scholars to hunch silently over a book, but BPL administrators envision the public library of tomorrow as a more active hub of collaboration and creation.

Rawn and Gayley said their design is intended to be flexible as the library continues to wade into the digital era.

“The next 40 years of the library is going to be all about the coexistence of traditional things you touch and feel with your hands and other things that are digitally based. It’s a real hybrid,’’ Rawn said.

“I hope 20 years from now, they’ll say we designed a library that has the flexibility to keep changing, because we think it will keep changing, radically.’’

Philip Johnson’s original vision for the addition to the Boston Public Library’s Central Branch

in Copley Square.

A rendering of the library’s current $78 million renovation project, designed by William Rawn Associates, Architects Inc.

Sidewalk reconstruction, as seen in January, is part of the effort to make the Boylston Street entrance more inviting and useful to the public.

Dan Adams can be reached at dadams@globe.com. Follow him on Twitter @DanielAdams86.