My must haves:
Ok I know you mentioned books already, but...Books, books, and more books. I have LOTS of books.
An easel for teaching on the carpet
pocket charts - a HUGE one and at least a couple smaller ones
Document camera (this is not a must have, but I use it a lot.)
Stuff for word work - fancy crayons (glitter, etc), letter magnets, letter stamps, letter tiles, etc.
Cardstock - I seem to use a ridiculous amount of cardstock. For printing math and ELA games from Teachers Pay Teachers or Scholastic $1 deal teacher resource books
Index cards (I use them SO much - words with phonics pattern, sight words, math skills...Kagan structures, small group, memory games, etc etc. I nearly always cut them in half so it's not quite as wasteful.)
Post it notes (I use them, the kids use them...they must always be on hand!)
My own paper cutter - I actually have 2 - 1 for home, 1 for my classroom
A pencil sharpener that actually works (Oh, and the kids CAN'T USE IT! lol)
Chart paper (big and small, lines and no lines) and markers
Individual whiteboards and markers for all the kids
Math manipulatives - if you're lucky these will come with your math program. I got pattern blocks, geoboards, clocks, and cubes with my math program. I bought coins and counters myself at Lakeshore.
Hope that helps.