An Alienist is what we would call a psychiatrist today. But in the terms of the Imperial Age game, he is a psionic parapyschologist.
The archetypical Alienist would be Dr. Seward from Dracula (sans psionic powers) or maybe even Hannibal Lector for an evil one.
This book also deals with the psychologically disturbed and how some of them can manifest wild psionic powers. So not quite the crazy one sees in Cthulhu by Gaslight, but more so than Masque of the Red Death.
I give this book credit for coming up for something very original. I think it is more closely tied the to campaign than say some other Advanced Classes like the Monster Hunter, but I can see this working quite well in say a Rippers game.
All the Imperial Age books are all well written and features art from the age, either public domain art and paintings as well as some original art. In all cases the art is very evocative of the time and very well done.
The books are all easy to read, with clean layout and font sizes. They can be printed with ease without killing your printer cartridge.