My most frequently used Jewish genealogy reference books are a dog-eared original copy of WOWW, Where Once We Walked (a gazetteer of Central and Eastern Europe) and a heavily annotated-and-highlighted copy of A Dictionary of Jewish Surnames from the Russian Empire.
Avotaynu publishes most essential Jewish genealogy reference books; WOWW was its first venture in 1991 with a revised 2004 edition.
I recall that the original didn't list my family's Belarus or Lithuanian shtetls. I wrote to Gary Mokotoff and asked why not? He asked for information, and I supplied various bits of evidence from other gazetteers and maps. The places were added to the new edition.
Since then, however, more additions and corrections have been received. Gary writes in his latest NU? WHAT'S NEW? newsletter that another edition is unlikely, so Avotaynu has established a web site to identify these changes, and WOWW users are invited to provide information.
This gazetteer is essential: In addition to listing more than 23,500 towns where Jews lived before the Holocaust, it also includes alternative names, longitude, latitude, pre-Holocaust Jewish populations, and other references to sources mentioning the town.
It was the first major use of the then-new Daitch-Mokotoff Soundex system, which helped researchers find the correct spelling of an ancestral towns even if they only knew what it sounded like and couldn't spell it.
A "nearby town" index also helps researchers identify towns near their ancestral shtetl, which may provide clues on where other family branches might have lived.
For more information about WOWW, click here.