diff --git a/server/market/bookrouter.go b/server/market/bookrouter.go
index c82b54b447..dbf9516003 100644
--- a/server/market/bookrouter.go
+++ b/server/market/bookrouter.go
@@ -8,6 +8,7 @@ import (
 	"encoding/json"
 	"fmt"
 	"sync"
+	"sync/atomic"
 
 	"decred.org/dcrdex/dex"
 	"decred.org/dcrdex/dex/msgjson"
@@ -150,7 +151,6 @@ type BookSource interface {
 type subscribers struct {
 	mtx   sync.RWMutex
 	conns map[uint64]comms.Link
-	seq   uint64
 }
 
 // add adds a new subscriber.
@@ -171,37 +171,33 @@ func (s *subscribers) remove(id uint64) bool {
 	return true
 }
 
-// nextSeq gets the next sequence number by incrementing the counter. This
-// should be used when the book and orders are modified. Currently this applies
-// to the routes: book_order, unbook_order, update_remaining, and epoch_order,
-// plus suspend if the book is also being purged (persist=false).
-func (s *subscribers) nextSeq() uint64 {
-	s.mtx.Lock()
-	defer s.mtx.Unlock()
-	s.seq++
-	return s.seq
-}
-
-// lastSeq gets the last retrieved sequence number.
-func (s *subscribers) lastSeq() uint64 {
-	s.mtx.RLock()
-	defer s.mtx.RUnlock()
-	return s.seq
-}
-
 // msgBook is a local copy of the order book information. The orders are saved
 // as msgjson.BookOrderNote structures.
 type msgBook struct {
-	name string
-	// mtx guards orders and epochIdx
-	mtx      sync.RWMutex
-	running  bool
+	running atomic.Bool // whether msgBook is initialized
+	name    string
+	subs    *subscribers
+	source  BookSource
+	baseID  uint32
+	quoteID uint32
+
+	// mtx ensures orders, epochIdx and seq are changed atomically with respect
+	// to each other.
+	mtx sync.RWMutex
+	// seq is tracking current(latest) order book version. See nextSeq for info
+	// on what version change actually means.
+	seq      uint64
 	orders   map[order.OrderID]*msgjson.BookOrderNote
 	epochIdx int64
-	subs     *subscribers
-	source   BookSource
-	baseID   uint32
-	quoteID  uint32
+}
+
+// nextSeq gets the next sequence number by incrementing the latest book version
+// counter. This should be used when the book and orders are modified. Currently,
+// this applies to these routes: book_order, unbook_order, update_remaining, and
+// epoch_order, plus suspend if the book is also being purged (persist=false).
+func (book *msgBook) nextSeq() uint64 {
+	book.seq++
+	return book.seq
 }
 
 func (book *msgBook) setEpoch(idx int64) {
@@ -219,7 +215,7 @@ func (book *msgBook) epoch() int64 {
 // insert adds the information for a new order into the order book. If the order
 // is already found, it is inserted, but an error is logged since update should
 // be used in that case.
-func (book *msgBook) insert(lo *order.LimitOrder) *msgjson.BookOrderNote {
+func (book *msgBook) insert(lo *order.LimitOrder) (note *msgjson.BookOrderNote) {
 	msgOrder := limitOrderToMsgOrder(lo, book.name)
 	book.mtx.Lock()
 	defer book.mtx.Unlock()
@@ -229,6 +225,7 @@ func (book *msgBook) insert(lo *order.LimitOrder) *msgjson.BookOrderNote {
 		//panic("bad insert")
 	}
 	book.orders[lo.ID()] = msgOrder
+	msgOrder.Seq = book.nextSeq()
 	return msgOrder
 }
 
@@ -245,14 +242,16 @@ func (book *msgBook) update(lo *order.LimitOrder) *msgjson.BookOrderNote {
 		//panic("bad update")
 	}
 	book.orders[lo.ID()] = msgOrder
+	msgOrder.Seq = book.nextSeq()
 	return msgOrder
 }
 
 // Remove the order from the order book.
-func (book *msgBook) remove(lo *order.LimitOrder) {
+func (book *msgBook) remove(lo *order.LimitOrder) (nextSeq uint64) {
 	book.mtx.Lock()
 	defer book.mtx.Unlock()
 	delete(book.orders, lo.ID())
+	return book.nextSeq()
 }
 
 // addBulkOrders adds the lists of orders to the order book, and records the
@@ -261,6 +260,7 @@ func (book *msgBook) addBulkOrders(epoch int64, orderSets ...[]*order.LimitOrder
 	book.mtx.Lock()
 	defer book.mtx.Unlock()
 	book.epochIdx = epoch
+	// book.seq starts with 0 here.
 	for _, set := range orderSets {
 		for _, lo := range set {
 			book.orders[lo.ID()] = limitOrderToMsgOrder(lo, book.name)
@@ -333,20 +333,17 @@ func (r *BookRouter) runBook(ctx context.Context, book *msgBook) {
 	// Get the initial book.
 	feed := book.source.OrderFeed()
 	book.addBulkOrders(book.source.Book())
+	book.running.Store(true) // can serve order book to clients now
 	subs := book.subs
 
 	defer func() {
+		book.running.Store(false) // can stop serving order book to clients now
 		book.mtx.Lock()
-		book.running = false
 		book.orders = make(map[order.OrderID]*msgjson.BookOrderNote)
 		book.mtx.Unlock()
 		log.Infof("Book router terminating for market %q", book.name)
 	}()
 
-	book.mtx.Lock()
-	book.running = true
-	book.mtx.Unlock()
-
 out:
 	for {
 		select {
@@ -376,9 +373,7 @@ out:
 				if !ok {
 					panic("non-limit order received with bookAction")
 				}
-				n := book.insert(lo)
-				n.Seq = subs.nextSeq()
-				note = n
+				note = book.insert(lo)
 
 			case sigDataUnbookedOrder:
 				route = msgjson.UnbookOrderRoute
@@ -386,10 +381,10 @@ out:
 				if !ok {
 					panic("non-limit order received with unbookAction")
 				}
-				book.remove(lo)
+				nextSeq := book.remove(lo)
 				oid := sigData.order.ID()
 				note = &msgjson.UnbookOrderNote{
-					Seq:      subs.nextSeq(),
+					Seq:      nextSeq,
 					MarketID: book.name,
 					OrderID:  oid[:],
 				}
@@ -405,7 +400,6 @@ out:
 					OrderNote: bookNote.OrderNote,
 					Remaining: lo.Remaining(),
 				}
-				n.Seq = subs.nextSeq()
 				note = n
 
 			case sigDataEpochReport:
@@ -449,7 +443,9 @@ out:
 					epochNote.TargetID = o.TargetOrderID[:]
 				}
 
-				epochNote.Seq = subs.nextSeq()
+				book.mtx.Lock() // book snapshot can be taken concurrently
+				epochNote.Seq = book.nextSeq()
+				book.mtx.Unlock()
 				epochNote.MarketID = book.name
 				epochNote.Epoch = uint64(sigData.epochIdx)
 				c := sigData.order.Commitment()
@@ -490,9 +486,9 @@ out:
 				}
 				// Only set Seq if there is a book update.
 				if !sigData.persistBook {
-					susp.Seq = subs.nextSeq() // book purge
-					book.mtx.Lock()
-					book.orders = make(map[order.OrderID]*msgjson.BookOrderNote)
+					book.mtx.Lock() // book snapshot can be taken concurrently
+					book.seq = book.nextSeq()
+					book.orders = make(map[order.OrderID]*msgjson.BookOrderNote) // book purge
 					book.mtx.Unlock()
 					// The router is "running" although the market is suspended.
 				}
@@ -559,21 +555,26 @@ func (r *BookRouter) sendBook(conn comms.Link, book *msgBook, msgID uint64) {
 	}
 }
 
+// msgOrderBook returns current (latest) order book snapshot.
 func (r *BookRouter) msgOrderBook(book *msgBook) *msgjson.OrderBook {
-	book.mtx.RLock() // book.orders and book.running
-	if !book.running {
-		book.mtx.RUnlock()
+	// Don't want to block client requests for too long, so return fast if order book
+	// isn't initialized yet instead of proceeding to wait on mutex below for who
+	// knows how long.
+	if !book.running.Load() {
 		return nil
 	}
+
+	book.mtx.RLock()
+	defer book.mtx.RUnlock()
+
 	ords := make([]*msgjson.BookOrderNote, 0, len(book.orders))
 	for _, o := range book.orders {
 		ords = append(ords, o)
 	}
 	epochIdx := book.epochIdx // instead of book.epoch() while already locked
-	book.mtx.RUnlock()
 
 	return &msgjson.OrderBook{
-		Seq:          book.subs.lastSeq(),
+		Seq:          book.seq,
 		MarketID:     book.name,
 		Epoch:        uint64(epochIdx),
 		Orders:       ords,