If it is only one or two pages, I suggest following Astronuc's advice. If more pages fell out, i have some suggestions..
Rebinding pages of a textbook is not difficult. A friend of mine who worked in his college library, showed me the technique. I repaired several of my college texts this way. Showing you would be the best method. But since I don't have a video of me doing this, the next best thing is to describe it here.
For reinserting pages into a spine:
I carefully cut the first page (attached to the cover) at the juncture where it meets the crease. Repeat this process on the last page (attached to the rear cover).
Once that is completed, you should be able to separate the spine from the hard cover.
Take care to save the little cloth pieces attached to the top and bottom of the spine (you may reuse them later).
If old glue is attached to the spine, gently peel it off or scrape it off with a single edged blade. Reinsert the pages in their proper sequence.
Line up the pages so that they are straight and square.
Clamp the book together between two firm flat surfaces (e.g. square pieces of wood or stiff metal), using C-clamps or ViceGrips. Clamp the book, such that the spine is protruding slightly from the two flat surfaces (to allow you access for regluing).
To increase the surface area to which the new glue will adhere, you can rough up the edge of the book with sand paper or coarse wood file. You can also score, the edge of the book spine, with diagonal cuts using a single edge blade. (like you might score the skin of a smoked ham before roasting).
Initially I tried Elmers glue but it became brittle and pages broke off again
I have since used flexible glues such as "acrylic latex caulking" or "flexible contact cement" and both work well.
When you have your pages squared up, firmly clamped in position, spine dressed to receive the glue; apply the glue evenly along spine. Work it into the scored sufaces with a flat edge (piece of flat cardboard or metal). Finally apply more glue and smooth across the whole surface of the spine with a metal spatula or your finger (wearing latex gloves during the gluing, makes cleanup easier). Allow glue to set up.
Glue the cloth pieces you saved earlier, back in place at the top and bottom of the spine.
Wrap the hard cover back around your book and glue a 1-2 in. width piece of heavy white rag paper to reattach the cover at the slits you made in the crease of front and rear covers.
For this cover attachment, a standard white adhesive, like elmers is fine.
You may fold this piece of white paper in half to facilitate gluing. Allow this glue to set.
Tuck the cloth pieces in at the top and bottom of your book and your done.
If you take your time, you will have a nicely rebound book.
