Actually, you don't need to go to a bookbinder - you could do it yourself if you want to get the materials.
The year before last when I was working in a school library, one of my jobs was to fix books that were in worse state than yours.
If your books are relatively new, the first thing I would do would be to talk to the bookshop you bought the book from or the publisher.
You could also get some advice by taking them to your local library as they might have someone who can give you some hands-on advice.
What I used to do was to apply some pvc-based liquid adhesive between the spine and the back cover. We used to use (I think it was called "Melamine", an archival grade wood and paper glue, its white and takes about 20-30 minutes to set. You can also apply the glue in the cavity under the paper at the crease/hinge of the bookcover if the damage isn't too great. With melamine, you need to either put a weight on the book or wrap elastic bands around the binding to ensure that the surfaces you want glued are together.
I also used a strip of clear contact down the inside crease of the back cover. Also, sometimes on the outside of the cover running down the crease/hinge of the book-cover.
Your piece of contact should be at least 4cm wide and should be at least 8cm longer than the spine of the book. When you align the contact with the book, hold the contact strip vertically and align the centre line of the strip with the crease between the book cover and the first or last page. (Often its a stiffer, heavier paper or card). Lay the contact precisely. If you've never used contact before, it can be fiddly, sticky stuff. Now that you've got your stip of contact down, you've now got two 4cm strips at either end. You need to trim part of this excess contact, but not all. With one of the strips, use scissors to cut down the centre line until you reach the crease of the book-cover. Then make a horizontal cut trimming the excess contact above the page , taking care not to lop off the strip above the book cover itself. Now you have a strip of contact 2cm wide and four cm long above the bookcover. Take the strip and applying a little bit of pull, wrap it over the outside of the bookcover. Then do the same for the other end of the book.
My apologies for the mangled instructions!