Hebrewbooks.org really cheered me up this afternoon with their list of new books
. Many items are important, particularly for the literature of the Geonim. For now I'll mention Otzar ha-Geonim, Benjamin Manasseh Lewin's crucial (if imperfect) compendium of Geonic writing on the Talmud. Also, Victor Aptowitzer's Mehkarim be-Sifrut ha-Geonim.
More later, I hope.