TBH I find the attitude expressed by
@Clint_L to be sort of presuming that monster write-ups should be written with a specific kind of DM in mind - a DM who (a) is experienced and (b) wants to have a bunch of their own lore rather than the default. This is something I can't agree with.
If you (general you) want monsters to have their own lore at your own table, you can already do that.
But a new DM, or a DM who doesn't want to have to come up with a bunch of lore,
shouldn't have to. There should be enough detail to "plug and play" a monster both in and out of combat at the very least. More intelligent monsters will necessarily require more write-up than, say, animal-like predators. Monster write-ups for specific settings can be more detailed.
Overall
I prefer a single monster to a page, with maybe a two-page spread for more significant monsters. If a single monster is getting more page count than that, it should be for lore, random tables, and maybe a variant stat block. If you're using a large font size, you might have to go to a two-page-spread per monster across the board.
Also, the entire description, both lore and stat block, should be focused towards using the monster in play. If page count permits, more lore can be provided that is not "play-focused", but if you've got maybe one column, or part of a column, you need to make the most of it.
I should say I'm quite partial to the designs that
@Sacrosanct has posted. I don't care for the way the Hacklopedia uses all those paragraphs - I'd prefer bullet points, myself - but I'm not prepared to opine on the content proper since I can't read it.
Monster Lore
If you're inexperienced or don't want to put in the work to come up with a bunch of lore, the monster lore should be sufficient to fit a monster into the "presumed default" setting for a game, such as a "kitchen sink" fantasy setting in the case of core book 5e. If the monster book uses different assumptions make sure to spell them out in the intro!
For instance, the Mercenary example in the Monster Overhaul is decent, since they're not so fantastic that a lot of work needs to be done just to give us, the reader, an idea of what mercenaries might be like.
Using the Monster
This is an area where I think 5e has done pretty badly, and I would say most versions of D&D and D&D-clones aren't too good at it. This includes some of the stuff
@Leatherhead brings up - suggestions for how to plug an encounter with that monster into your game without only revolving around the combat stat block. ENWorld's A5E does a decent job at this, I'd reckon.
For instance, the Mercenary example in the Monster Overhaul is decent. I don't care for the way the random tables don't appear to have any relationship to any of the text on the rest of the page - I'm assuming, though, that there's another page in the spread missing that fills that particular gap.
Monster Stat Blocks
In general, I'm a fan of the 4e/5e stat block format, which basically gives you everything you need to run the monster in a combat encounter right on the spot. (I prefer the 4e and nouveau-5e style of not having to include digging through your PHB for spell descriptions, although the WotC implementation of the nouveau-5e style does leave a good deal to be desired.)
So, for instance, the Mercenary example in the Monster Overhaul is not a great stat block to my mind, since there's a huge gap between its "HD" line and the next actual game statistic line.
I tried to "walk the talk" with the 4e-like I'm working on at the moment. The thread is
over here, but for convenience of reference, I'm reproducing the monster pages in this thread. The monster is the
aboleth Deep One, which will likely be one of the most complicated monster stat blocks.
The lore page (which will probably either be in a two-page spread with a full-page picture or with the gameplay page):
The gameplay page:
The stat block page:
If the gameplay and stat block pages are on the same two-page spread, then, in theory, everything you need at minimum to run the monster on the spot is right there at a glance when you open up the monster book, with the lore on the previous two-page spread for when you want to refer to it. The lore includes ecology, lairs, and even a "choose your own lore" element.
The Deep One's stat block also reinforces the brief snippet on its tactical behaviour: it (1) tries to daze or sicken opponents to leave them vulnerable to (2) its attempts to dominate them, and then (3) if that succeeds it can bring them permanently under its control. This three-step process also gives player characters time to try to stop it! It also has off-turn actions it can take that either reinforce why fighting one in its lair is dangerous or reinforce why it always wants to fight with thrall allies.
