The Tavern

I propose that when joining Sif Muna, that you get some piety for your already learned magic skills.

The rationale is that if you're planning on joining Sif Muna, the current setup encourages you delay training your magic skills until after you've found her altar. Or if you're not training magic skills when you do find the altar, to delay joining her until you've finished training whatever non-magic skills you were training and are ready to switch over to magic skills again.

The current setup can also put some characters in an awkward position: a DEWz might already have all of the spells in the Book of Minor Magic castable by the time he finds Sif's altar, which makes it awkward to train magic skills to gain piety!

Agreed. I've done some Transmuter characters that take her primarily for access to a lot of spells with plans on being a quote-unquote 'hybrid' with a UC focus. The fact that her piety doesn't take prior skills into account leads to my early game on those characters being: Pretend you are a monk, train UC and a little fight/dodge...find altar...train Tm only...when otherwise that wouldn't be optimal play for a Tm. [I call that character design max-buffs. Focuses Blade Hands, but gets enough other magic skills to get all the buffs online, Phase Shift, Ozo's Armour, etc.]

Agreed.

Something of the inverse of this problem: I would also like it if Sif gave some way for characters who didn't already have a spell book and a spell memorized to gain piety.

She could grant piety for training Spellcasting, which would work in a pinch. We could also do something like the old Ashenzari vaults: put a randart spellbook with a single low-level spell next to the altar.

Well currently "Sif Muna does not accept worship from the ignorant," meaning you cannot worship her if you haven't memorized at least one spell.

The piety gain mechanic for Sif could probably use some tweaking. Also the message I just quoted is pretty opaque; a player could reasonably assume that they need to gain levels or raise intelligence to worship Sif, when in fact simply knowing any spell (regardless of power or success rate) is what's required.

What about someone who converts to Sif Muna late in the game, already having a lot of spell skills? This sounds like it could result in being showered with quite a lot of books instantly, unless you scale differently for prior skill training.

Or just have the piety get quickly diminishing returns for high levels of training that taper off to a maximum. Probably you shouldn't be able to start off with more than ** or so piety this way.

Ah, a bit like converting between good gods then? That would be fair.

already does

In wizmode, setting piety to 200/200 (the max possible) gives you four book gifts. I'm a little surprised; I thought you'd only get one. I imagine it wouldn't be hard to code an exception to get only a single book. Or maybe 4 is okay, or there should be a limit of **.... or ****.. something. I don't really think it would be a problem to get 4 books on joining, but I'm not really invested in what precisely happens with a late-game switch.

Currently if you happen to be playing a troll with all non-magic skills at 27 and all magic skills at 0, and join Sif, and happen to have enough unspent skill points to train all the magic skills to 27 when you kill a monster (or reload, but that just causes a segfault instead currently), you will get 330 book gifts.
You do run out of disciplines for randart spellbooks partway through this. But other than that there are no problems - 330 is well below the item limit, and that is the absolute worst-case scenario that would ever happen in any case.

So I do not see the problem with getting spellbooks upon joining.

I think this is a good idea if Sif is your first god, but not for people who have already had another religion. You shouldn't be able to worship another god while effectively getting Sif piety (or at least the important part of it).