I've just finished David Bell's What Nuns Read. (marking off books from my to-read list) Most of the book is a complete list of manuscripts that can be attributed to English nunneries, preceded by a short analysis of finances, literacy, and other issues. The analytic chapters have some good hard numbers, including incomes of women's monasteries, comparisons of those incomes to the annual wages of laborers, information on costs of books, and so forth. For English nuns, Bell concludes, there is better evidence of literacy and book-reading than has generally been supposed. Even in Latin, in spite of frequent medieval complaints that nuns are not sufficiently Latin-literate. He points out, also, that similar criticisms are being directed at monks at the same time, so it's not as if late medieval monks were preserving some better state of literacy and composition than nuns were.
What about nuns on the continent? As far as I know there is no comparable study and book-list for any region. Alison Beach has done some work on women and book production in Germany (more for the to-read list), and there are several other scholars who have worked on nuns' writing and artistic production there. Fiona Griffiths' book on Herrad of Hohenburg makes a case for a high level of literacy, indeed of theological sophistication, on the part of Herrad, and perhaps others of her nuns. This case might be quite exceptional, however. The various nuns and communities which produced spiritual treatises (such as Helfta), may also be quite exceptional. So, some communities might be especially noted for learning: what about other houses, especially smaller and poorer ones?
Any monastery had to have some books. In the first place, a monastery required books to perform the liturgy. Bell notes that over half of the surviving manuscripts of English nunneries were liturgical. One might expect similar figures for medieval survivals elsewhere. Except that in Catholic Europe, the liturgy was substantially revised by the Council of Trent in the mid-16th century. Older books were in many cases discarded as the new ones came in, especially older books that had no particular artistic merit and therefore didn't seem worth keeping. Thrifty religious sometimes recycled such outmoded books as end-papers for new ones.
In the second place, a community following the Benedictine rule was supposed to distribute one book annually to each member, and this was to be read over by the nun or monk during the year and then returned. By this logic, even a small house ought to have at least as many non-liturgical books as it had members.
For the Iberian kingdoms there seems to be not much known about writing, literacy, or book ownership among nuns at any point in the Middle Ages. One house I've done research on kept large numbers of medieval charters, but its archive has no medieval books. Another, in contrast, has many liturgical books and a few other devotional books still in its own archive. The dislocations of the 19th and 20th centuries might have caused books to be lost or destroyed, or moved into larger collections. Some translations of the Benedictine rule into Catalan might have been made for a women's monastery, for example. There is, I think, a lot that could be done on this topic, though I don't know if I am the right person to do it.
A parting note: a lengthy article on the Benedictine congregation in Catalonia indicated that copies of the orders constitutions were kept in the two largest nunneries associated with the congregation. Why them particularly? Is that somehow a testament to the quality of the nuns' libraries or record-keeping?
Apologies for the somewhat disconnected thoughts, I am still feeling my way on these questions.