diff --git a/src/book.rs b/src/book.rs
index 3de5c6c..d04a568 100644
--- a/src/book.rs
+++ b/src/book.rs
@@ -781,6 +781,30 @@ impl OrderBookManager {
         book.apply_delta(delta)
     }
 
+    /// Apply a WebSocket `book` update to a managed book.
+    ///
+    /// This is the preferred way to ingest `StreamMessage::Book` updates into
+    /// the in-memory order books (avoids rebuilding snapshots via per-level deltas).
+    pub fn apply_book_update(&self, update: &BookUpdate) -> Result<()> {
+        let mut books = self
+            .books
+            .write()
+            .map_err(|_| PolyfillError::internal_simple("Failed to acquire book lock"))?;
+
+        if let Some(book) = books.get_mut(update.asset_id.as_str()) {
+            return book.apply_book_update(update);
+        }
+
+        // First time we've seen this token; allocating the key and book is part of warmup.
+        let token_id = update.asset_id.clone();
+        books.insert(token_id.clone(), OrderBook::new(token_id, self.max_depth));
+
+        books
+            .get_mut(update.asset_id.as_str())
+            .ok_or_else(|| PolyfillError::internal_simple("Failed to insert order book"))?
+            .apply_book_update(update)
+    }
+
     /// Get a book snapshot
     /// Returns a copy of the current book state that won't change
     pub fn get_book(&self, token_id: &str) -> Result<crate::types::OrderBook> {
diff --git a/tests/no_alloc_hot_paths.rs b/tests/no_alloc_hot_paths.rs
index 6e7465e..51b3bc9 100644
--- a/tests/no_alloc_hot_paths.rs
+++ b/tests/no_alloc_hot_paths.rs
@@ -5,7 +5,7 @@ use std::hash::{Hash, Hasher};
 use std::str::FromStr;
 
 use chrono::Utc;
-use polyfill_rs::{OrderBookImpl, Side};
+use polyfill_rs::{book::OrderBookManager, OrderBookImpl, Side};
 use rust_decimal::Decimal;
 
 thread_local! {
@@ -164,3 +164,54 @@ fn no_alloc_apply_book_update_existing_levels() {
     book.apply_book_update(&update).unwrap();
     guard.assert_no_allocations();
 }
+
+#[test]
+fn no_alloc_book_manager_apply_book_update_existing_levels() {
+    let asset_id = "test_asset_id";
+    let manager = OrderBookManager::new(100);
+    manager.get_or_create_book(asset_id).unwrap();
+
+    // Warm up the internal book with initial levels (allocations allowed).
+    manager
+        .apply_delta(polyfill_rs::types::OrderDelta {
+            token_id: asset_id.to_string(),
+            timestamp: chrono::Utc::now(),
+            side: Side::BUY,
+            price: Decimal::from_str("0.75").unwrap(),
+            size: Decimal::from_str("100.0").unwrap(),
+            sequence: 1,
+        })
+        .unwrap();
+    manager
+        .apply_delta(polyfill_rs::types::OrderDelta {
+            token_id: asset_id.to_string(),
+            timestamp: chrono::Utc::now(),
+            side: Side::SELL,
+            price: Decimal::from_str("0.76").unwrap(),
+            size: Decimal::from_str("100.0").unwrap(),
+            sequence: 2,
+        })
+        .unwrap();
+
+    let update = polyfill_rs::types::BookUpdate {
+        asset_id: asset_id.to_string(),
+        market: "0xabc".to_string(),
+        timestamp: 10,
+        bids: vec![polyfill_rs::types::OrderSummary {
+            price: Decimal::from_str("0.75").unwrap(),
+            size: Decimal::from_str("200.0").unwrap(),
+        }],
+        asks: vec![polyfill_rs::types::OrderSummary {
+            price: Decimal::from_str("0.76").unwrap(),
+            size: Decimal::from_str("50.0").unwrap(),
+        }],
+        hash: None,
+    };
+
+    // Warm up TLS access before measuring (defensive).
+    let _ = allocation_count();
+
+    let guard = NoAllocGuard::new();
+    manager.apply_book_update(&update).unwrap();
+    guard.assert_no_allocations();
+}