I'll make it easier than that. Follett and Bound to Stay Bound (BTSB). Yes, there are more distributors and yes there is more to buy than books, but I'm just trying to cover the basics here. If you stick with these two at first you can make this pretty easy and rewarding for all.
Let's pretend this is my first year at the library and I just got $ to buy books for the year. Since this is my first year, I don't have any teacher or student requests and my only guide is the school's learning goals which basically, when it comes to reading, says we want to improve reading.
I open an account with Follett and learn how to run a collection analysis. It's not hard and there are friendly people that will walk you through it. Unless there is something glaring at you, let's just say you need to order good books.
The first thing you do is log into Titlewave, Follett's ordering system. They've change it a bit recently and I'm elementary and I don't want to do a million screen shots, so this will be pretty basic. But what I would do is click on "Collection Development" then click on "Books" and a screen with a bunch of blanks comes up. I would click on the box next to K-3 and 3-6. Then I would click the box next to "Publisher's Hardcover." Then I would click the blue button at the top left that says "Search Books." Now you will see some books. Make sure the "Sort By:" drop down menu is set for "Most Popular." And you are set. These will be the 2500 currently most ordered titles by other librarians for there grade levels. See how awesome this is? Now you'll want to go through and pick the ones most appropriate for your population and what your student's and teachers will like. You'll also want to know which titles to get more than one copy of. But those things will become clearer the more you do this.
Now you're not really going to order a bunch of publisher's hardcovers from Follett. You want as many of these as are available from BTSB. So open up a new tab in your browser and log into BTSB and go down the Titlewave list and add them to a list in BTSB. Anything that has a red line next to it is something you already have in your library. Anything that doesn't show up in BTSB, you can go back to later.
Now I'm not a total weasel. I still order from Follett too. If they have it in FollettBound and not at BTSB, I'll probably order it, especially if it's not something that needs the BTSB treatment. I don't order graphic novels from Follett anymore for example because they only last about five minutes before they start falling apart. But a non-fiction book that will only be checked out once a year by teachers on Groundhog Day or something, it doesn't really matter about the binding. If I'm in a hurry I may do the same search but click on the "FolletBound Sewn" and "FollettBound Glued" boxes rather than the "Publisher's Hardcover" box and that's fine as well. Those are good books and in some way the covers look better than the BTSB ones. On balance, this can be an overall cheaper option as well. But if they are high-circulation titles they just won't last as long.
Every once is a great while I'll have a teacher request a book that I can only find in paperback on Titlewave. What I usually do with those is process them myself and put it in the Professional collection, which only teachers have access to.
BTSB is great because they give you the paper covers to use (I use them for bulletin boards) and they stamp the Dewey information into the spine, no stickers. After doing this job for a while you'll realize there is a certain segment of humanity that cannot resist peeling off any kind of sticker, no matter how well glued down. That's one less sticker per book to have to re-process later.
One more example. Let's say I'm in a high school and I need to order new books just for the science section. On that book screen, I would click on the boxes next to 5-8 and YA (to get a good range of reading levels). Then I would click on "FolletBound Sewn," "FollettBound Glued," and "Publisher's Hardcover." I'd scroll down to "Dewey Range:" and put in 500 to 599.999 and run my search. Here's where you would also want a curriculum map in front of you so you'd make sure to get science books not only of interest, but that went along with what you'r students are actually studying. But since what comes up is the most popular books ordered by other librarians, it's a good bet these are good titles to check out. You'll see right away what you can go ahead and get library bound and the ones that you can't you can decide it you want to try to get them form BTSB now or wait and see if they come available in better bindings later.
For your first year, if a teacher asks you for a book and it's not at BTSB or Follett, then tell them you can't get it. Don't be a superhero trying to track down out of print or out of stock books just yet.
I could say a whole lot more, but I think this gives you an overview of a good way to get started and make the process pretty painless. Actually, since I love making lists, I enjoy this part of it the most next to actually getting to open up the boxes of new books when they come in!
Throw me more questions and comments about this so I can clear anything else up you may want to know.