Re-bound Hardback Library Books 

These are generally Trade Edition books that were sent off to a company who specialized in binding books for libraries.  The covers were removed and the text was bound into a new set of durable and sometimes ugly covers called "buckram".  The covers on these books are often beige or tweed in color.  Sometimes a picture of the original book cover or one of the illustrations from inside the book was used on the front cover.  Some have monochrome pictures of the original cover art.  I highly recommend these books for your children or other young relatives - they last forever and you can buy them very cheaply.  There are many variations of re-bound books and I like to find unusual ones to add to my collection.  See some examples below.

Perma-Bound Library Books 

These are Trade Edition paperbacks in which the texts were removed from the covers, the covers were fortified with cardboard or plastic then laminated, and then the texts were sewn back into the covers.  A cheap, economical way for libraries to avoid the costlier GLB books which, in the mid-1980's, cost $5.39 each.  I do not recommend perma-bound books for collectors unless you are simply in need of the title until a Trade or GLB Edition comes along.  I routinely see these advertised at on-line auctions as "ex-library hardbound" and I see collectors unwittingly buying them thinking they are getting a true GLB book at a great price.  Always ask about the elephant in the oval!
An interesting re-bound library copy
A typical re-bound library book
One can find Three Investigators books in a wide variety of library bindings - some quite worthless, others worth a small fortune.  Especially in the 1960's, many libraries filled their shelves with the regular Three Investigators Trade Edition. Librarians quickly found out that these did not last long in young hands due to the poor construction of the books - the textblocks easily detach from the covers.  So, libraries often had them re-bound to hold up to the rigors of heavy use.  Their other option was to purchase the rather expensive Gibraltar Library Binding [GLB] Edition which was an extremely well-made and durable product.
Gibraltar Library Binding (GLB) Edition

These are true hardbound books published by Random House for use in libraries.  All of the original 43 Three Investigators titles can be found in the GLB Edition as well as the eleven Crimebusters titles.  Several things distinguish the GLB Edition from the Trade Edition:  1) a sturdy, stitched binding; 2) most GLB's have a smooth semi-gloss finish; and 3) the GLB logo.  The logo is always found on the back cover and it consists of an oval encircling the silhouette of an elephant standing on an open book over the words "Gibraltar Library Binding".  Most GLB books do not have a list of titles on the back cover but there are several exceptions.  Also, like the Trade Edition, GLB's printed before 1975 will contain the blue cemetery endpapers BUT, probably because of the different manufacturing process, they are "reversed".  Plain white eps replaced the blue eps.
Because the GLB books are so durable, I highly recommend them to collectors who want their children to read The Three Investigators series but don't want their fragile and valuable Trade Editions damaged.  As collectors know, GLB titles #29 - #43 are the only hardbound format in which these titles are available - they were not printed in a hardbound Trade Edition. Consequently, they are highly sought after by collectors and they can command extremely high prices on the secondary market.  Depending on condition, the last 15 titles in GLB usually sell for $15.00 to $50.00 each on-line.  Near mint and uncirculated examples often sell for well over $100.00.  In one extreme case on eBay in September of 2000, several of these high-number GLB's brought prices ranging from $102.50 up to an all-time record of  $611.00 for a copy of #41 Creep-Show Crooks.  On rare occasions, a collector may find a GLB Edition which never saw use in a library and doesn't have any of the associated markings - many times a teacher or librarian would purchase these books to use as classroom gifts or contest prizes.  Even the first 28 GLB titles can fetch very high prices if they are in uncirculated condition.

Usually with series books, the last title or two are considerably harder to find than are the earlier titles in a series.  This is simply because later titles were in print for a shorter period of time.  It is my opinion that this is NOT the case with The Three Investigators GLB books.   I would consider GLB #43 Cranky Collector to actually be one of the more common GLB titles as compared to GLB #35 Kidnapped Whale or GLB #41 Creep-Show Crooks which seem to show up less often in the secondary market.

There is also a Library Edition of the 1998 - 2000 Dodge Issue (first eleven titles only).  These are hardbound glossy cover books almost identical to the paperback Trade Edition of this issue.  There is no specific wording on these books identifying them as a Library Edition but that is how they are being marketed through  You can learn more by visiting the page on the Dodge Issue on this site.
The four Find Your Fate books are also available in a hardbound 
library format but the little oval logo contains the words "Reinforced Library Binding" (RLB). 

Each of the GLB books can be found sporting the original cover art found on the Trade Edition books.  One can also find eight of the earlier titles reissued in GLB in the early 1980's with the new Adragna cover art and white endpapers.  For example, #21 Haunted Mirror can be found in GLB with the Jack Hearne cover art AND it can also be found with the later Adragna art with Hitchcock's silhouette in the upper-right corner.  The eight early-title-alternative-art GLB titles (others may exist) are:  

#4 Green Ghost
#5 Vanishing Treasure
#13 Crooked Cat
#17 Singing Serpent
#20 Monster Mountain
#21 Haunted Mirror
#22 Dead Man's Riddle
#23 Invisible Dog

The "Elusive Adragna Eight" GLB's are very difficult to find:
GLB spines
The GLB logo.
"Reversed" eps as found in the GLB's.