Reddit reviews Monster Manual: Core Rulebook III v. 3.5 (Dungeons & Dragons d20 System)

Basically you need the Player's Handbook, the Dungeon Master's Guide, and the Monster Manual. There are also "premium" versions of these books printed in 2012 that include many years worth of errata, and you should try to get those if you can.

The three core books are enough to play, but of course Wizards of the Coast published a very large number of supplements to the game that add more rules, systems, and advice for DM's to use and more options, classes, and spells for players. One of these is the Dungeon Master's Guide II, which includes a large bit of advice for DM's, more magic items, and some new rules and systems, including a detailed system for building towns. It was a fine supplement, but is certainly not essential for running a campaign, and I wouldn't even describe it as the best 4th book for a new dungeon master to buy.

Actually, if you're just starting out DMing, the best 4th book to get would probably be a pre-published adventure, or even a mega-adventure or adventure path. The Sunless Citadel is an excellent adventure for 1st-level PC's, though you should check first whether anyone in your group has played through it before. As for mega-adventures, both The Red Hand of Doom and Age of Worms have excellent reputations, though the latter was published in Dungeon magazine which makes it hard to find a copy.

I am a new DM too (we've played maybe 5 sessions?)

I am using the 3 core rulebooks:

The Player's Handbook
http://www.amazon.com/Dungeons-Dragons-Players-Handbook-Rulebook/dp/0786928867

The Dungeon Master's Guide
http://www.amazon.com/Dungeon-Masters-Guide-Rulebook-Dungeons/dp/0786928891/

Monster Manual
http://www.amazon.com/Monster-Manual-Rulebook-Dungeons-Dragons/dp/078692893X/

The players themselves only need the Player's Handbook.

There is no reason to use other books when you're starting out. All they'll do is add complication to an already complicated system. When I was at school and joined a group for just a few sessions to learn what was going on, one guy had several other books he was pulling spells and items from and it only allowed him to make the rest of the party irrelevant. He was able to pull things from other books to circumvent most plot points that required other characters' skills and abilities, and it was just a bad time for everyone involved.

As everyone I'm playing with is new (me too) - we chose to do a premade series of dungeons to get the feel of it. I chose the Sunless Citadel and pretty much completely changed the story within a few sessions and nobody else knows or cares because we're all enjoying it.

I'm a 3.5 man myself. The 2e guys and I will have our disagreements but we're all pretty much unanimously agreed that 4e is not the way D&D was meant to be played.

Player's Handbook 3.5e

Dungeon Master's Guide 3.5e

Monster Manual 3.5e

However, every D&D group will play differently anyway. You will NEED the PHB and the DGM. You will also need a full set of poly-dice (d4, d6, d8, d10, d12, d20, d%). Stories, additional material, models, boards, scenery is all optional. We tend to switch between zero physical assets, just talking (usually only if free-roaming and/or lack of confrontation, etc.), and a white-board where areas are drawn in erasable marker, with warhammer miniatures.

You want the Monster Manual that [looks like this] (http://www.amazon.com/Monster-Manual-Rulebook-Dungeons-Dragons/dp/078692893X/ref=sr_1_2?ie=UTF8&qid=1451094951&sr=8-2&keywords=3.5e+DnD+Monster+Manual) and the Dungeon Master's Guide that [looks like this] (http://www.amazon.com/Dungeon-Masters-Guide-Rulebook-Dungeons/dp/0786928891/ref=sr_1_2?ie=UTF8&qid=1451095029&sr=8-2&keywords=3.5e+DnD+Dungeon+Master%27s+Guide).

Of course, they're out of print now, so you'll probably only find them used for various prices.

It's my physical copy of 3.5 edition but I bet there's one online in PDF somewhere. If you just want shots of the two monsters here you go. If you can't find it online, I'll happily send you art from other monsters in my 3.5 or 4th edition Monster Manuals :)