I am looking for a book in German called approximately "Sextips for
Schwule Menner" (sex tips for gay men).

The book "Sextips für schwule Männer," by Stephan Niederwieser (ISBN
3-86187-836-4) may be purchased from Amazon's German site:

Amazon.de: Sextips für schwule Männer 
http://www.amazon.de/exec/obidos/ASIN/3861878364/ref=ase_schildnet/302-0502728-7469664

Other online sources:

Libri.de: Sextips für schwule Männer
http://www.libri.de/shop/action/productDetails?artiId=2489957

Buch.de: Sextips für schwule Männer
http://www.buch.de/buch/04422/589_sextips_fuer_schwule_maenner.html

Bookzilla.de: Sextips für schwule Männer
http://www.bookzilla.de/shop/action/productDetails?aUrl=90006951&artiId=2489957

ABEbooks.de: Sex Tips für schwule Männer
http://www.abebooks.de/servlet/BookDetailsPL?bi=331707414

Here you'll find a price comparison of different merchants:

Buchpreis24.de
http://www.buchpreis24.de/cgi-bin/search.pl?isbn=3861878364

If you prefer not to order from a German-language site, you can place
an order through the ABEbooks site in the United States:

ABEbooks.com: Sextips für schwule Männer
http://dogbert.abebooks.com/abe/BookDetails?bi=283011684

My Google search strategy:

Google Web Search: "sextips für schwule männer" isbn
://www.google.com/search?hl=en&q=%22sextips+f%C3%BCr+schwule+m%C3%A4nner%22+isbn

I hope this will enable you to purchase the book. If anything is
unclear or incomplete, or if a link doesn't work for you, please
request clarification; I'll be glad to offer further assistance before
you rate my answer.

Best regards,
pinkfreud