Disclaimer: I'm fairly new to the game system chronologically, but I've done something of a deep-dive since finding it.
300BP is indeed, not a whole lot for books, especially if you're hoping to speed advancement along. There are some sample spreads in AR5, p.72 in the example covenant builds - at 300BP you're looking somewhere between low and medium. Increasing the size and quality of the library is likely one of the priorities for a new covenant, and many covenants do have a minimum season of covenant service per year that PCs might want to spend expanding the library.
Most-everyone likes their PCs advancing faster rather than slower, so I personally like to go with a smaller, but higher BP per book library. I'd aim for around 200BP in Art Summae across 7 books, with a 3/4 spread in techniques/forms, with no single book under 26BP, no initial overlaps, and the vast majority of them at quality 15+, barring 31BP books whose art levels prohibit that quality level, with maybe one or two higher level, lower-quality summae. Also worth noting the especial utility of the Level 6, Quality 21 Form Summae - in a single season, a mage can go from 0xp to level 6 in the skill, getting him the +2 defensive form bonus and ensuring he can open an apprentice to that Form without deficiency.
Example Spread, Arts:
1 Level 18, Quality 13 Summae (Form) -31
2 Level 16, Quality 15 Summae (1 Technique, 1 Form or Technique ) -31 2
1 Level 14, Quality 12 Art Summae (Form) - 26
1 Level 11, Quality 16 Art Summae (Technique) - 27
2 Level 6, Quality 21 Art Summae (2x Forms) - 272
For Abilities, the only true 'must have' IMO right off the ground is Magical Theory, and it'll be useful to nearly everyone in the covenant, barring the one Bonisagus Lab Rat who's starting with too high of an ability score to benefit from it anyway.
Level 6, Quality 15 Magical Theory Summae - 33BP
The remaining 67 BP I typically use across 7-8 Tracati, but you can cut this down by 3-4 and add in another Level 5, Quality 20 (35BP) ability Summae that fits the theme of the game or covenant. Order of Hermes Lore, Fairie Lore, or Penetration are popular choices here and are also useful across a wide variety of campaigns. Arcane/Lore skills should be prioritized generally - a covenant that isn't fighting for its life should be able to have a fairly successful early expedition to acquire a variety of more mundanely accessible ability-related tracti and summae for the library.