That's BECMI - in BECMI the DM decides what your first few spells are (with the book suggesting Read Magic+either Sleep or Magic Missile).
This continues for the first four levels, with the M-U's mentor giving them one spell for every level - IIRC they suggest that Shield should be the second one?
All spells beyond that need to be recorded into your spellbook from either scrolls or stolen spellbooks using Read Magic.
B/X is different, of course - you only know as many spells as you can cast, and ALWAYS know as many spells as you can cast. Level one is a hard choice, yeah, but I'd stick with Sleep or Magic Missile if only because by the point you get enough scrolls to make Read Magic worthwhile you're probably level 2.
OD&D is also worth noting for the way it handled things - every time you get access to a new spell level, you get a spellbook with all the spells in that level. In other words, you always know all the spells for the levels you have available.
In addition, any M-U can create magic items at any time - scrolls are 500gp/spell level and take a week/spell level to craft, for instance. However, you need to cast Read Magic once for every two scrolls you want to cast.
Supplement 1: Greyhawk changed things around a bit, making Intelligence actually matter by giving you a minimum/maximum number of spells known for each level and a chance to randomly determine which.
Being able to record spells from scrolls wasn't a thing until AD&D, I think, which is probably where Mentzer got it from.
And yeah, Read Magic is stupidly useful if you have scrolls. If you don't then it's not really useful, but then again in an ideal session you wouldn't need to use Sleep or Magic Missile either so it's hard to say which is the best choice for a first-level M-U.
Assuming that you get the choice, that is.>>44619374
IIRC B/X goes the OD&D route in requiring Read Magic to read spell scrolls but allowing anyone to read the Protection: X scrolls.