diff --git a/CHANGELOG.md b/CHANGELOG.md
index 3c185e0..1625fdd 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,10 @@
 # Changelog
 
+## 1.2.0
+
+- Add `Unit::BookPayment` for booking transfers between Unit accounts.
+- Allow `belongs_to` relationships to specify the JSON:API `type` sent to Unit.
+
 ## 1.1.1
 
 - Publish releases automatically to RubyGems.org via OIDC trusted publishing when `version.rb` changes on `main`.
diff --git a/README.md b/README.md
index 98bb7a2..c2dccb7 100644
--- a/README.md
+++ b/README.md
@@ -95,6 +95,20 @@ debit_card = Unit::IndividualDebitCard.create(
     shipping_address: customer.address,
     tags: { externalUserId: my_user_id }
 )
+
+# Book a transfer between two Unit accounts
+other_account = Unit::DepositAccount.create(
+    deposit_product: 'checking',
+    customer: customer
+)
+
+book_payment = Unit::BookPayment.create(
+    amount: 10_000, # in cents
+    description: 'Transfer between accounts',
+    idempotency_key: "book-payment-for-#{my_user_id}",
+    account: deposit_account,
+    counterparty_account: other_account
+)
 ```
 
 > [!TIP]
diff --git a/lib/unit-ruby.rb b/lib/unit-ruby.rb
index 5353713..8ff1798 100644
--- a/lib/unit-ruby.rb
+++ b/lib/unit-ruby.rb
@@ -29,6 +29,7 @@
 require 'unit-ruby/application_form'
 require 'unit-ruby/atm_location'
 require 'unit-ruby/authorization'
+require 'unit-ruby/book_payment'
 require 'unit-ruby/ach_counterparty'
 require 'unit-ruby/customer_token_verification'
 require 'unit-ruby/customer_token'
diff --git a/lib/unit-ruby/book_payment.rb b/lib/unit-ruby/book_payment.rb
new file mode 100644
index 0000000..e646dd7
--- /dev/null
+++ b/lib/unit-ruby/book_payment.rb
@@ -0,0 +1,25 @@
+module Unit
+  # A transfer of funds between two Unit accounts
+  class BookPayment < APIResource
+    path '/payments'
+
+    attribute :amount, Types::Integer # Amount to transfer in cents.
+    attribute :description, Types::String # Description of the payment. Max of 50 characters.
+    attribute :transaction_summary_override, Types::String # Optional. Overrides the transaction summary. Max of 100 characters.
+    attribute :idempotency_key, Types::String # Optional
+    attribute :tags, Types::Hash # Optional
+
+    attribute :direction, Types::String, readonly: true
+    attribute :summary, Types::String, readonly: true
+    attribute :status, Types::String, readonly: true
+    attribute :reason, Types::String, readonly: true
+    attribute :created_at, Types::DateTime, readonly: true
+
+    belongs_to :account, class_name: 'Unit::DepositAccount'
+    belongs_to :counterparty_account, class_name: 'Unit::DepositAccount', type: :depositAccount
+    belongs_to :customer, class_name: 'Unit::IndividualCustomer'
+    belongs_to :counterparty_customer, class_name: 'Unit::IndividualCustomer'
+
+    include ResourceOperations::Create
+  end
+end
diff --git a/lib/unit-ruby/util/api_resource.rb b/lib/unit-ruby/util/api_resource.rb
index 16a27c8..3a12876 100644
--- a/lib/unit-ruby/util/api_resource.rb
+++ b/lib/unit-ruby/util/api_resource.rb
@@ -98,8 +98,13 @@ def resource_type
 
     # Creates an association to a related resource
     # This will create a helper method to traverse into a resource's related resource(s)
-    def self.belongs_to(resource_name, class_name: nil)
+    #
+    # @param resource_name [Symbol] the name of the relationship
+    # @param class_name [String] the class of the related resource
+    # @param type [Symbol] the JSON:API type sent for the relationship, when it differs from the relationship name
+    def self.belongs_to(resource_name, class_name: nil, type: nil)
       class_name ||= resource_name.to_s.camelize
+      type ||= resource_name.to_s.camelize(:lower).to_sym
 
       define_method(resource_name) do
         relationship_id = relationships.dig(resource_name, :data, :id)
@@ -111,7 +116,7 @@ def self.belongs_to(resource_name, class_name: nil)
 
       define_method("#{resource_name}=") do |resource|
         relationships[resource_name] = {
-          data: { type: resource_name.to_s.camelize(:lower).to_sym, id: resource.id }
+          data: { type: type, id: resource.id }
         }
       end
     end
diff --git a/lib/unit-ruby/version.rb b/lib/unit-ruby/version.rb
index cd69ddc..82394a9 100644
--- a/lib/unit-ruby/version.rb
+++ b/lib/unit-ruby/version.rb
@@ -1,3 +1,3 @@
 module Unit
-  VERSION = '1.1.1'
+  VERSION = '1.2.0'
 end
diff --git a/spec/features/book_payment_spec.rb b/spec/features/book_payment_spec.rb
new file mode 100644
index 0000000..f27f220
--- /dev/null
+++ b/spec/features/book_payment_spec.rb
@@ -0,0 +1,52 @@
+require 'spec_helper'
+require 'securerandom'
+
+RSpec.describe Unit::BookPayment do
+  before do
+    establish_connection_to_api!
+  end
+
+  let(:new_individual_customer) do
+    Factory.create_individual_customer
+  end
+
+  let(:deposit_account) do
+    Factory.create_deposit_account(new_individual_customer)
+  end
+
+  let(:counterparty_deposit_account) do
+    Factory.create_deposit_account(new_individual_customer)
+  end
+
+  it 'serializes the counterparty account relationship as a deposit account' do
+    payment = Unit::BookPayment.new
+    payment.counterparty_account = Unit::DepositAccount.new.tap { |account| account.id = '10000' }
+
+    expect(payment.relationships[:counterparty_account]).to eq(
+      data: { type: :depositAccount, id: '10000' }
+    )
+  end
+
+  it 'creates a book payment between two accounts' do
+    Factory.fund_deposit_account(deposit_account, 10_000)
+    expect(Unit::DepositAccount.find(deposit_account.id).available).to be >= 5_000
+
+    payment = Unit::BookPayment.create(
+      amount: 5_000,
+      description: 'Funding',
+      transaction_summary_override: 'Transfer to savings',
+      idempotency_key: SecureRandom.uuid,
+      account: deposit_account,
+      counterparty_account: counterparty_deposit_account,
+      tags: { tag1: 'value1' }
+    )
+
+    expect(payment.type).to eq 'bookPayment'
+    expect(payment.status).to eq('Sent'), "Book payment was #{payment.status}: #{payment.reason}"
+    expect(payment.amount).to eq 5_000
+    expect(payment.description).to eq 'Funding'
+    expect(payment.tags[:tag1]).to eq 'value1'
+    expect(payment.account.id).to eq deposit_account.id
+    expect(payment.counterparty_account.id).to eq counterparty_deposit_account.id
+  end
+end
diff --git a/spec/spec_helper.rb b/spec/spec_helper.rb
index 3b3085d..df7ff3f 100644
--- a/spec/spec_helper.rb
+++ b/spec/spec_helper.rb
@@ -48,6 +48,20 @@ def self.create_deposit_account(customer)
     Unit::DepositAccount.create(deposit_product: 'checking', customer: customer)
   end
 
+  # Simulates an incoming ACH credit to fund an account (sandbox only)
+  def self.fund_deposit_account(deposit_account, amount)
+    Unit::APIResource.connection.post(
+      '/sandbox/payments',
+      {
+        data: {
+          type: 'achPayment',
+          attributes: { amount: amount, direction: 'Credit', description: 'Sandbox funding' },
+          relationships: { account: { data: { type: 'depositAccount', id: deposit_account.id } } }
+        }
+      }
+    )
+  end
+
   def self.create_debit_card(customer, deposit_account)
     random_user_id = rand(10**9).to_s.rjust(9, '0')
 
diff --git a/spec/version_spec.rb b/spec/version_spec.rb
index 9e12a83..979f52a 100644
--- a/spec/version_spec.rb
+++ b/spec/version_spec.rb
@@ -2,7 +2,7 @@
 
 RSpec.describe Unit do
   it 'returns the correct version' do
-    expect(Unit::VERSION).to eq '1.1.1'
+    expect(Unit::VERSION).to eq '1.2.0'
   end
 end