TiKZ: Why is a line visible between 2 fitted nodes with inner/outer sep =0?
Probably the problem is rendering in viewer (I use Sumatra). If I magnify image obtained with your code, this line disappear. As cure for this viewers discrepancy try the following:
\documentclass{book}
\usepackage{tikz}
\usetikzlibrary{fit}
\begin{document}
\begin{tikzpicture}%
[every node/.style =
{fill=blue, draw=blue, % <-- added "draw"
inner sep=0mm, outer sep=0mm,
minimum width=5cm, minimum height=5cm
}
]
\node (first) at (10,10) {};
\node[fit=(first.north) (first.south),anchor=east] (second) at (first.west) {};
\end{tikzpicture}
\end{document}
Edit:
In the Percusse answer is explanation why in your case a thin line between adjacent nodes is visible. Added option draw
in above MWE cause, that nodes overlap for thickness of border lines:
consequently there is no more place for image viewer artifacts. Above picture is generated with:
\documentclass{book}\usepackage{tikz}
\usetikzlibrary{fit}
\begin{document}
\begin{tikzpicture}[
test/.style = {fill=#1, draw=#1, draw opacity=0.5,
inner sep=0pt, outer sep=0pt,
minimum width=5cm, minimum height=5cm}
]
\node (first) [test=blue] {};
\node (second) [test=red,
fit=(first.north) (first.south),
anchor=east] at (first.west) {};
\end{tikzpicture}
\end{document}
This effect is generally known as the conflation artifact and it has nothing to do with TikZ. It is a well-known artifact by the graphic designers and they avoid this by avoiding shapes sharing perfectly aligned edges or shapes filling a cut-out exactly.
See for example rendering problems here (wait a bit after you click to see the changes). Similarly graphics.SE has lots of relevant questions and probably the Hillary logo is the most voted one: Is there a reason Hillary Clinton's logo has hidden notches?
So the typical solution is to make things overlap slightly.