French: up to 1900; French history, Russo-French history, the War of 1812, anything related to Napoleon.
‍
Books about Russian theatre and ballet from the early 20th century (Diaghilev’s Ballets, Russian Seasons, Serge Lifar…).

In all languages, of interest: First editions of works, editions with autographs of classic authors of world literature, and poets. If it’s from the 17th or 18th century, then it was printed during the author’s lifetime.

Modern (20th century) first editions (Harry Potter, The Lord of the Rings, The Little Prince, Hemingway, Orwell…).

Scientific discoveries, geographical discoveries, and travel accounts — described in their own time and era, the first accounts of these findings (America, Australia, the Arctic… Herberstein, Cook, Livingstone, Pallas, etc.).

Natural science editions with a large number of hand-colored engravings, 17th–18th centuries (Birds of America, Birds of Australia, botanical atlases).

Book graphics, books with illustrations by 20th-century artists and architects, their theoretical works published in small print runs (usually noted, up to 1,000 copies) — Picasso, Dalí, Le Corbusier, Chagall, Miró, Basquiat…

Culinary books — the older, the better.