John Lee (Introduction to Smooth Manifolds) seems to use some package that Detexify doesn't know for his curly letters. Here are some examples:
Does anyone know how to generate these letters?
According to mathalpha's documentation, it seems that this is Mathtime Pro 2 script, which is a commercial (i.e., not free) font.

Isn't this one? Maybe you should search for some different font for the exact shape, but...
mtpro2package. If you have access to the full (non-free) version of this package, you would load it with the optionmtpcaland generate the letters by writing (in math mode)\mathbcal{B},\mathbcal{D},\mathbcal{L}, etc. – Mico Feb 01 '21 at 18:25