What is the biggest book written in braille on the open market today?
Most Braille books are not made for anything that you would call the open market. They are too large to store and need to be handled carefully. Only few people ever read braille books, those who don't have enough vision to read large print books, yet can read braille. Even though braille terminals are still rather expensive, it's still cheaper to read text on them then to print and store books.
The few braille books that are produced are for libraries, rather than the open market. The libraries that handle this aren't public ones, they restrict circulation to vision-impared patrons for multiple reasons. They need to avoid getting the books damaged, they need to provide a valuable service from limited funds. Most importantly if they only serve vision-impared patrons, that lets them use the special exemption in the copyright protection laws when producing braille editions of books. Without such a special exemption, it would be a nightmare to get licences from copyright owners, most of whose time isn't worth negotiating about printings in a single copy.
The largest (and probably only) library in Hungary that loans braille books is the library of the Magyar Vakok és Gyengénlátók Egyesülete (MVGYOSZ). Although they restrict circulation of braille books and audiobooks, their catalog is freely accessible on the internet.
From that catalog you can tell that the biggest braille book in the library is the first three volumes of the four volume novel series Csendes Don (Тихий Дон, known in English as And Quiet Flows the Don) by Mihail Alekszandrovics Solohov (Михаил Александрович Шолохов, 1905–1984), at 174 volumes shorthand braille total. If you count this as three separate books at 58 volumes each, then it's still tied for the longest among several books that are 59 or 58 volumes braille with contractions (or between 20000 and 30000 words), most of which also novels. The only non-fiction book of that size is the diary of Széchenyi István in Hungarian translation.