The AAVSO Manual for Visual Observing of Variable Stars (ISBN 978-1-878174-00-0) is a comprehensive guide to variable star observing. Included is a lot of information and tips on how to make variable star observations and report them to the AAVSO. The Manual is available online in .pdf format. Translations of the Manual are available in several languages.
AAVSO Manual for Visual Observing of Variable Stars  - March 2013 Edition (7.8 MB)
The latest revisions to the Manual are summarized here: 2013ManualUpdates.doc 
In an effort to make the Manual more useful to our members and observers world-wide, we have undertaken a project to translate the manual into different languages. Thanks to the assistance of several ambitious and hard-working volunteers in various countries, we now offer translations of the Manual in the following languages:
Arabic (in progress)
Chinese  - "Simplified Chinese" version translated by Cheng Simiao, January 2012.
"Traditional Chinese" version translated by Tao Fan-Lin December 2012.
French  - translated by Dominique Naillon, January 2006.
Hungarian - translated by Péter Molnár and Dr. Laszlo Kiss, January 2010.
Italian  - translated by Dr. Giuseppe Bianco, December 2008.
Japanese  - translated by Seiji Tsuji, May 2009.
Persian  - translated by F. Bahrani and her colleagues at the Dr. Hesabi Institute, February 2012.
Polish  - translated by Andrzej Grabowski and Jerzy Speil, December 2007.
Portuguese  - translated by David Duarte Cavalcante Pinto and members of CEAAL, April 2011.
Spanish  - translated by Jaime Garcia, April 2006.
Turkish  - translated by Tuğrul Uşşaklı, Sabahattin Bilsel, and Haldun Menali, December 2006.
Since we are considering revisions and a new way to present the Visual Observing Manual. the following version has not been formatted in the same way as the other languages were. Instead, they it is being made available in a draft state until the new presentation becomes available.
Russian  (11.1 MB .doc) - translated by Artemy Kilin, October 2011.
Finnish Manual on variable star observing  by Mika Luostarinen