Reddit reviews The Physics of the Manhattan Project

There's the Los Alamos Primer, which is an original publication of the Manhattan Project.

Another one is The Physical Principles of Thermonuclear Explosive Devices.

There's also a newer book The Physics of the Manhattan Project which has a website that has FORTRAN code and Excel spreadsheets for calculations.

The physics actually isn't all that exotic. Even the engineering aspects of the bomb's design is fairly straightforward. The complications are in the actual production of the materials and building the thing to work as designed. While a lot of this stuff is probably technically classified, it's been published publicly. The US Government makes it a policy not to comment on the content of these publications, as if they do, it's a giveaway that it contains something classified.