Part 6: Monsters

At 15 pages, you get another very considerable load of monsters, which brings the total count to something around 200. Lots of classic D&D monsters like basilisks, displacer beasts, the same six giants as from the 3rd edition monster manual, invisible stalker, wraith, and wyvern. What is quite interesting is the notable abscence of some other creatures that are now widely seen as fundamental parts of D&D. Beholders did get added with the Companion set in 1983, but there's no mind flayers, no aboleths, no yuan-ti, no mimics, no kuo-toa, no drow, and no demons and devils. Mind flayers, kuo-toa, and drow had been around in AD&D since 1978 and yuan-ti had been created also in 1980 also by David Cook. Aboleths first appeared later in 1981 for AD&D (also created by Cook). The Fiend Folio for AD&D appeared in the same year, and to my knowledge none of its creatures ever made it into BECMI, which means no slaads or githyanki either.
There are actually quite a number of monsters unique to BX/BECMI of which some sneakily made it into 3rd edition. Though those might actually have been picked from the very late Mystara monster book for AD&D.

Part 7: Treasure

There are six more pages of magic items, and since the descrptions are all very brief, there's actually quite a lot of them. +3 weapons do appear on the random item charts, but the chance for them is extremely rare. The vast majority of magic weapons is +1 with various special abilities.
There are two whole pages for creating intelligent swords, a concept that never appealed to me and which I've only seen used once in Baldur's Gate II.

The Protection from Magic scroll is quite interesting. It creates a 10 feet radius area around the user that last 10 to 40 minutes and into which no spells or spell effects can enter, and also not leave. Casting spells inside the area on other targets inside the area appears to be still possible, though.
The Ring of Regeneration seems very powerful as it heals 1 hp per round. Which would be 60 hp per turn and few player characters would ever have much more than that. If you got one, you can get your whole party back to full health within an hour or so.

In the Basic Set, wands could only be used by wizards and staves only be used by clerics, which was a nice idea. Here in the Expert Set that rule doesn't apply and each wand or staff is identified as either being usable by wizards or by clerics. Since both Sets were written and printed at the same time, I don't understand why they would do such a thing.

The other items are mostly quite nice and interesting, but there's almost nothing among them that just increases any of your characters numbers. They are mostly about doing new things, not doing existing things better.

Part 8: Dungeon Master Information

The idea of Ability Checks, which was hidden in the very back of the Basic Rules, is repeated here again in a much more prominent position.

Wizard and cleric players get the option to research new spells. Such spells require approval of the GM and creating them takes 1000 gp and 2 weeks for every spell level.

At 9th level they can also create magic items. Which finally allows them to make scrolls instead of playing russian roulette with scrolls they find on adventure. Which by now doesn't matter anymore since a 9th level cleric can simply raise anyone who gets killed by a scroll or remove any other curse recieved from looking at a cursed scroll. Creating scrolls takes as long as researching spells but only half the gold. Sure, scrolls are great, but it doesn't really seem like a good option unless the party is taking a break for a few months. Other items regularly take several months to make. Yeah, you can kinda do it, but it doesn't really seem practical.

One of the major new things about the Expert Set is that 9th level characters can now buy and build castles. Why exactly do you have to be 9th level to do that? If you got the money, why can't you buy them or hire workers? There's a list of prices for several types of walls and towers, but some of the notes just don't make any sense. A tower with a 20 feet base and 30 feet height costs 15,000 gp. So far so good.
The cost for a tower is normal as long as its height does not exceed it's base width, but building higher costs twice as much and it can not be higher than twice the base width. So how much does a 30 feet high tower cost now?
The whole section on building castles takes up a little bit more than half a page. There's not really any mention of what you'd do with a castle, except that you can have one. Settlers can be attracted to the land around your castle and "will pay taxes (10gp per year or whatever the DM decides)". Which to me reads like "10 gp per year or whatever". How many settlers? How do you get them? What do you do with them?
This part of the game is completely underdevelopped and there is nothing there but "At 9th level you can build a castle and it costs this much". My suspicion is that castles really only exist at this point to have some way how players can spend their huge piles of gold they carried out of the dungeons they had been to to get XP from them. Money doesn't really have any use in Expert once you cashed in your XP for getting it.

Another page is about designing a wilderness. Which I guess might be somewhat useful to new GMs who don't have any clue how to begin that process. Draw a map, place some towns, place some dungons. New encounter tables for wandering monsters at higher levels and in the wild, but much more it really doesn't have to say.

There's a sample map that later grew into the Mystara setting and a map on ship combat, and that's the end of the book.

The Expert Set is new spells, new items, and new monsters, and in that regard it delivers pretty well. In as far as expanding the dungeon crawling game into a game of wilderness explorations it's kind of a dud. There isn't really anything much useful there, other than introducing the concept of hex maps instead of square maps.
Not that I am a fan of domain government subsystems like ACKS does it. This is all stuff I would personally just handwave in my own campaigns. But from all the hype the Expert Set often gets, I had expected something much more exciting. It's a perfectly servicable expansion to advance Basic characters above 3rd level, but that's really all it is.