I read Steve Mitchell's Notes on principal bundles and classifying spaces (pdf). There is a theorem: Let $G$ be any topological group, $H$ an admissible normal subgroup. Then there is a homotopy-fibre sequence $BH\overset{Bi}{\rightarrow }BG\overset{% B\rho }{\rightarrow }B(G/H)$ where $i:H\rightarrow G$ is the inclusion and $% \rho :G\rightarrow G/H$ is the quotient map.
I need a reference (an article or book, not lecture notes) for this theorem with ''admissible hypotesis''.