@lostinearth, Serious suggestion, you might try putting them in the freezer (in a bag so they don't get wet) because the cold will make them shrink, and that might be enough to get them out. 

That's a good suggestion from Fantasia. As well as doing that, if they don't come out immediately, you could also take a butter knife (with a totally blunt and flat blade, I mean) and slide it in very gently between the books and the casing and see if you can loosen them a bit more that way. There'll be a solution!!  

Now I'm curious to know which set you have My boxed set (1980 Fontana Lions) is delicate, too, so I have to be careful with them; they do tend to be tight. (Getting them back in the box successfully is another problem.)

Couple of recommendations as a librarian (which you probably already know): Don't try reefing on the spine--there's no surer way of damaging the books; if you can, find something to safely grip an individual book, since fingers are often awkward, and merely add to the congestion.

As @courtenay mentioned above, using a blunt knife between the books might help--maybe try one of the middle books as well; if you can get one out, the rest ought to come more easily.  Also, press gently on the top of the box, so that the sides bow out a bit.  If you can figure out where it is binding, it might give you something to go on.

@fantasia's suggestion is a good one, especially if the books have swollen from dampness. I've only ever put absolutely drenched books in myself, but it does work wonders.

Good luck with your endeavours!

Once your books are as dry as you can get them, I suggest using a narrow pair of metal tongs to try to pull one out, or lever one out using a wide screwdriver or chisel, etc.

If these ideas are still unsuccessful, you are faced with a choice - the books or the box!  If you choose to cut into the box, use a craft knife (Stanley knife) and cut into the edges of the side it normally stands on, so it will show less when you repair it.

Best wishes! My 1970s set is sometimes hard to lever out, but they are loose enough to tip slightly and then grab the top of one spine.

So sorry to hear of this misfortune! (I have the 1970's Collier box set and have trouble getting them out occasionally).

I'd suggest using something thin and stiff that comes in a right-angle shape, like a hex key wrench or a bent coat hanger. If you maybe inserted the bent end in between two books, you could turn it sideways once you reached the back of the box, thus hooking around one so you can pull it out. 

OR, try sticking something like thin, stiff leather or rubber in one of the cracks and then pulling out. Possibly it will cling to the book and hold on? 

Hope these help!

@fantasia They have made their journey to the freezer!!!

I have tried to attach a file of the book set I have, it may be to big though. Thanks everyone for the advice ?