There are a few examples.
From Storm King's Thunder
There is a specific room in one of the major locations which has the following:
Chapter 10 says, in Room 25 - Library (p. 212):
Five rocks have the following wizard spells inscribed on them: antimagic field, conjure elemental, fabricate, legend lore, and stone shape. The rocks function like pages of a spellbook, but each weighs 600 pounds. A character who has a spellbook can record these spells in it.
These are not spellbooks, but can provide spells for wizards to copy.
...and there are a few books that aren't strictly spellbooks but do contain wizard spells:
From Out of the Abyss
In one of the major locations, an important NPC has the following item:
Chapter 4 says, in the Treasure section under "Finding Droki" (p. 69):
Pages of lizard-skin parchment held together between two leather covers from two different books. The pages are full of mad scribbles and arcane formulas, but a character proficient in Intelligence (Arcana) can study them for 1 hour to discover that they constitute a spellbook containing the Tenser’s floating disk and feign death spells.
This is not quite a typical spellbook but provides a couple spells in a manner similar to one.
From Icewind Dale: Rime of the Frostmaiden
There is a book found in a major location:
Chapter 5, in Room G22 - Chamber of the Codicil, contains The Codicil of White, a magic book whose description appears in "Appendix D: Magic" (p. 317):
The codicil was written by followers of Auril as a primer on her worship. The first page is a title page with the snowflake symbol of Auril on it. The remaining pages describe various priestly rituals and ceremonies in chilling detail. Nestled among these descriptions is a spell that wizards can learn (frost fingers, described later in this appendix) [...]
While it is a book, it is explicitly not a spellbook, simply containing a spell that wizards can transcribe.
From Dungeon of The Mad Mage
In addition to those above, there is a stone statue with a stone likeness of a spellbook that a spell can be copied from:
On level 7, in the courtyard of Maddgoth's Castle (p. 102)
Statue. In the center of this open-air courtyard, surrounded by gardens of glittering, pebble-sized crystals, stands a 9-foot-tall statue of a plump human wizard (...) reading from a spellbook. (...)
Carved into the pages of its open spellbook is the Mordenkainen's Sword spell. A wizard can take a rubbing of the spell and copy it into a spellbook.