I was a member of Children's Book of the Month Club when my oldest son was one. I was able to get many books at "ok" prices. Mind you, this was in the day before e-bay and half.com. (Well, maybe in the early days, but before I was savvy to those great resources). I thought the shipping was kind of high, but once I fulfilled my commitment, I was able to take advantage of the monthly deals. Plus, all the "free" books for becoming a member were pretty nice too.
I just recently signed up again, this time to get some older kid books for my son who is now 8. When you sign up, you get to choose 8 books for $2. I was able to get quite a few books, because the selections I made were all sets with 3 or more books each. I ended up with 25 books that would have cost around $165 for just under $30, which is the $2, plus the cost of shipping. I used some of the books for Christmas and Birthday presents, and still have some put away for gifts or other surprises.
By joining, you commit to purchase 5 other books at regular club prices over the next 2 years. At least that is the way it was when I signed up.
However, you do need to read and follow all of their instructions pretty closely. I just ordered 3 books and for some reason, they didn't count towards my requirement. Also, if you are not on top of things, you will be sent a monthly selection you may not even want that is totally overpriced.
I think if you sign up, get a really good deal on your initial books, then over the 2 year period you have to purchase your required books, watch for good deals, make sure they will count towards your requirement, buy them and get out, it could work out to be a pretty good deal and a great way to build your library of kids books.