Generally, Adventist books on a particular topic are shelved with other books on that topic. For instance, John Harvey Kellogg would be in the R section with biographies of other doctors.
BX 6100-6199 is for books on Adventist history, general Adventist topics, biographies of individual Adventists, and related materials.
Ellen White books are downstairs in the wooden bookcase beside the Reference section.
The Adventist Pioneer collection is in the Heritage Room on the second floor.
Check the library catalog (search box below) if you have a particular book or author in mind!