Not all the books were burned. You have to remember that library had a collection of works in the tens of thousands, and there were a few days after the Institut für Sexualwissenschaft was raided before the book burning happened, and people were going in to have a look around.
Even after the burning there were still a few stacks left. Many of these were smuggled or looted, or ended up being seized by the Nazi state to be sold (like they did with any art they deemed to be degenerate).
So consequently there are parts of the collection being recovered today.
Despite what we're being told, the institute wasn't just a transgender clinic or even a gay clinic; it was a holistic sexology research institute. Certainly there were collected studies on transgender surgery, but this isn't lost knowledge because it was the 1930s and they had a brute force approach (they didn't invert anything, they literally sliced it all off and then dug a hole). Interviews with transgender patients and case studies would also have been in the archives, along with scientific journals. They published their own scientific journal, Jahrbuch für sexuelle Zwischenstufen ("Annual for sexual intermediaries"), looking at gay and intersex people, and were preparing documents for a medical conference about intersex conditions.
Also all the paperwork surrounding the World League for Sexual Reform, which was campaigning for things we'd recognise as progressive causes today (civil marriage, birth control, destigmitisation of illegitimate children) and also regressive causes (eugenic birth selection). Additionally there were gay interest magazines in the archive like Der Eigene ("The Unique"), but plenty of other interest topics
including books of poetry who were written by people who'd go on to be
semi-important Nazis who wanted to increase genetic purity. There was also art collections, historic works (e.g. a folio of very gay seeming poems and writings by Friedrich Schiller) and an ethnographic collection of artefacts from around the world. Also the other stuff you'd expect from a place like that, like the collected works of Freud. A women's rights organisation used to meet there as well to discuss various forms of women's lib, and I don't think much is known about them because that all got burned. Also pornography. I can't attach pictures right now but you can see there were collections of pornography in the photographs the Nazis took of them flipping through the books, which again would make sense for a sexology researcher.
There were also a litany of personal effects from clients (it appears it operated as a sexual health clinic, counselling centre and social hub) including the personal writings of a German doctor who turned up there one night on the eve of his wedding and donated it all, because he was going to commit suicide because he couldn't face marrying a woman but felt he had no other choice. Bleak.