Seven books as follows:

Mystery of The Moss-Covered Mansion, number 18 in the series
Copyright, 1971, Library of Congress Catalog Card Number 77-155244
XX Chapters on 177 pp., illustrated with a frontispiece plus five at full page.
   Condition: 60% tight.

The Ghost of Blackwood Hall, number 25 in the series
Copyright 1948
XXV Chapters on 216 pp., only one illustration; that is, the frontispiece.
  Condition: name, number and address in ink on the front endpapers.

The Clue of the Leaning Chimney, number 26 in the series.
Copyright, 1949
XXV Chapters on 212 pp., only one illustration; that is, the frontispiece 
   Condition: name in ink on the verso of the flyleaf of the front endpapers.

The Witch Tree Symbol, number 33 in the series
Copyright, 1975, Library Catalog Card Number 75-1580
XX Chapters on 179 pp., illustrated with a frontispiece and five at full page.
   Condition: 55% tight, name on the paste-down of the front endpapers.

The Haunted Showboat, number 35 in the series.
Copyright 1957
XX Chapters on 184 pp., illustrated with a frontispiece, four at full page and one half-page that extends over.
  Condition: 13/16" corner fold and name on verso of the fly leaf of the front endpapers.

The Secret of the Golden Pavilion, number 36 in the series
Copyright, 1959
XX Chapters on 184 pp, illustrated with a frontispiece and four at full page plus one at half-page that extends from page 82 over to page 83.
   Condition: 50% tight.

The Mystery of the Fire Dragon, number 38 in the series
Copyright 1961
XX Chapters on 182 pp., illustrated with a frontispiece, four at full page and one half-page that extends over the next page.
  Condition: a number 38 in ink on the front of endpapers, page 139 with a 5/16" bottom corner fold.  Pull off mark on the front cover.

NOTE: (all books) unless otherwise indicated above, text is unmarked, no tears, no folds, clean.  See all photos for the exterior condition.  Spine is firm, all books stand on its bottom edge.  All leaves accounted for on all the books.