Magic-users will be interested in compiling a library to facilitate spell research.¹⁾ The procedure for assembling a library is somewhat unclear; this page rectifies that.

The library must be assembled in a highly private area. Lack of security may invite robberies, as with any other collection of high value.

The 10x cost multiplier for assembling a library listed on DMG 115 must be paid per spell level. Once the payment has been made for a spell level, the requisite materials have been assembled for researching spells of that level. Therefore, there are nine payments which may be made before the library is complete.

If you do the math, the per-level costs (not cumulative) end up being:

This reflects the additional 9x weekly base cost of the spell level, multiplied by the spell level (since preparations take 1 week per spell level).

Therefore, a complete library costs 570,000 gp worth of materials.²⁾

If appropriate books or like materials are recovered on an adventure, the DM may permit those materials' value to be put toward the required cost of one of the library's spell levels. The DM may decide that a book is too common to be useful for 4th level spell research, for example, and only allow it to be used for assembling the 1st, 2nd, or 3rd spell level library.

If the magic-user compiles their library over time in this way, the amount they've contributed to a spell level should be carefully tracked on their character sheet. Make sure to record each specific book from adventures which have been added! Some variation is needed. An entire spell level's worth of books only on geology won't do. However, the books used do not all need to be about magical subjects – they should cover the whole spectrum of knowledge.³⁾

Another magic-user's library can be used, but this does not reduce the base and variable costs.⁴⁾ It only eliminates the x10 multiplier for compiling the books required for research.