Secondary litterature of Leibniz - Gregory Brown's great bibliography
Leibniz-Bibliographie - maintained by the Gottfried-Wilhelm-Leibniz-Bibliothek. Contains material from 1991
References
Editions of Leibniz's works - by Donald Rutherford
A short list of litterature about Leibniz from Leibniz-Archiv
German books of Leibniz available - from Amazon.de
Leibniz-bibliography - from Dolphin logic
Secondary litterature of Leibniz - from Dolphin logic
Leibniz's published works - from Dolphin logic

Google book search of all-text books by Leibniz
Google book search of all books on Leibniz (most can be previewed)
Clinton Tolley's list of Leibniz's texts available in Googlebooks