diff --git a/src/SIL.Machine/QualityEstimation/BookConfidence.cs b/src/SIL.Machine/QualityEstimation/BookConfidence.cs
new file mode 100644
index 00000000..c9567fef
--- /dev/null
+++ b/src/SIL.Machine/QualityEstimation/BookConfidence.cs
@@ -0,0 +1,40 @@
+using System;
+
+namespace SIL.Machine.QualityEstimation
+{
+    public static class BookConfidence
+    {
+        public static readonly double LowBookConfidenceThreshold = 0.42;
+
+        /// <summary>
+        /// Determines whether a book confidence is unusually low, i.e. below
+        /// <see cref="LowBookConfidenceThreshold"/>.
+        /// </summary>
+        /// <param name="confidence">
+        /// The book confidence, calculated as the geometric mean of the segment confidences. Must be between 0 and 1,
+        /// inclusive.
+        /// </param>
+        /// <param name="bookId">The book id. Reserved for future book-specific logic; not currently used.</param>
+        /// <param name="model">The model name. Reserved for future model-specific logic; not currently used.</param>
+        /// <returns>
+        /// <c>true</c> if <paramref name="confidence"/> is below <see cref="LowBookConfidenceThreshold"/>; otherwise,
+        /// <c>false</c>.
+        /// </returns>
+        /// <exception cref="ArgumentOutOfRangeException">
+        /// <paramref name="confidence"/> is not between 0 and 1, inclusive.
+        /// </exception>
+        public static bool IsBookConfidenceUnusuallyLow(double confidence, string bookId = null, string model = null)
+        {
+            if (!(confidence >= 0 && confidence <= 1))
+            {
+                throw new ArgumentOutOfRangeException(
+                    nameof(confidence),
+                    confidence,
+                    "The book confidence must be between 0 and 1, inclusive."
+                );
+            }
+
+            return confidence < LowBookConfidenceThreshold;
+        }
+    }
+}
diff --git a/tests/SIL.Machine.Tests/QualityEstimation/BookConfidenceTests.cs b/tests/SIL.Machine.Tests/QualityEstimation/BookConfidenceTests.cs
new file mode 100644
index 00000000..145758fc
--- /dev/null
+++ b/tests/SIL.Machine.Tests/QualityEstimation/BookConfidenceTests.cs
@@ -0,0 +1,59 @@
+using NUnit.Framework;
+
+namespace SIL.Machine.QualityEstimation;
+
+[TestFixture]
+public class BookConfidenceTests
+{
+    [Test]
+    public void IsBookConfidenceUnusuallyLow_AtThreshold()
+    {
+        Assert.That(BookConfidence.IsBookConfidenceUnusuallyLow(BookConfidence.LowBookConfidenceThreshold), Is.False);
+    }
+
+    [Test]
+    public void IsBookConfidenceUnusuallyLow_MinConfidence()
+    {
+        Assert.That(BookConfidence.IsBookConfidenceUnusuallyLow(0.0), Is.True);
+    }
+
+    [Test]
+    public void IsBookConfidenceUnusuallyLow_MaxConfidence()
+    {
+        Assert.That(BookConfidence.IsBookConfidenceUnusuallyLow(1.0), Is.False);
+    }
+
+    [Test]
+    public void IsBookConfidenceUnusuallyLow_WithBookIdAndModel()
+    {
+        using (Assert.EnterMultipleScope())
+        {
+            Assert.That(
+                BookConfidence.IsBookConfidenceUnusuallyLow(0.3, "MAT", "facebook/nllb-200-distilled-1.3B"),
+                Is.True
+            );
+            Assert.That(
+                BookConfidence.IsBookConfidenceUnusuallyLow(0.9, "MAT", "facebook/nllb-200-distilled-1.3B"),
+                Is.False
+            );
+        }
+    }
+
+    [Test]
+    public void IsBookConfidenceUnusuallyLow_Negative()
+    {
+        Assert.Throws<ArgumentOutOfRangeException>(() => BookConfidence.IsBookConfidenceUnusuallyLow(-0.5));
+    }
+
+    [Test]
+    public void IsBookConfidenceUnusuallyLow_GreaterThanOne()
+    {
+        Assert.Throws<ArgumentOutOfRangeException>(() => BookConfidence.IsBookConfidenceUnusuallyLow(1.5));
+    }
+
+    [Test]
+    public void IsBookConfidenceUnusuallyLow_Nan()
+    {
+        Assert.Throws<ArgumentOutOfRangeException>(() => BookConfidence.IsBookConfidenceUnusuallyLow(double.NaN));
+    }
+}