This book is not as comprehensive as Ethnologue (and its statistics will be out of date because it was published in 2004), but is very easy to navigate as all the languages are listed in simple alphabetical order:
In-depth, peer-reviewed summaries on a range of topics. Click "Browse by Subject" and select "Language Families/Areas/Contact" for entries on language families
The Concise Oxford Dictionary of Linguistics
Dictionary Plus Linguistics
Encyclopedia of Rhetoric
International Encyclopedia of Linguistics
The Oxford Companion to the English Language
The Oxford Encyclopedia of Morphology,
Encyclopedia of Semiotics