The format definition
\DeclareFieldFormat{titlecase}{\MakeSentenceCase{#1}}
makes all titles in sentence case, which isn't what you want. Titles need to be printed according to both the entry and field types. For example, with the title field we need to handle @article and @book entries differently. With @inproceedings entries we need to handle the title and booktitle fields differently.
To do this we can redefine the title bibmacro to print the title field of @article and any @in* entry type in sentence case. Taking the original definition found in biblatex.def:
\DeclareFieldFormat{sentencecase}{\MakeSentenceCase{#1}}
\renewbibmacro*{title}{%
\ifthenelse{\iffieldundef{title}\AND\iffieldundef{subtitle}}
{}
{\ifthenelse{\ifentrytype{article}\OR\ifentrytype{inbook}%
\OR\ifentrytype{incollection}\OR\ifentrytype{inproceedings}%
\OR\ifentrytype{inreference}}
{\printtext[title]{%
\printfield[sentencecase]{title}%
\setunit{\subtitlepunct}%
\printfield[sentencecase]{subtitle}}}%
{\printtext[title]{%
\printfield[titlecase]{title}%
\setunit{\subtitlepunct}%
\printfield[titlecase]{subtitle}}}%
\newunit}%
\printfield{titleaddon}}
Alternatively we can identify book-like entries directly and apply sentence casing to everything else. This is trickier because many more types qualify as book-like references and titles for these sources are printed by more than just one macro. In biblatex.def these include: title, booktitle, maintitle, journal, periodical and issue. To avoid redefining all of these, you can redefine the titlecase format instead.
\DeclareFieldFormat{titlecase}{\MakeTitleCase{#1}}
\newrobustcmd{\MakeTitleCase}[1]{%
\ifthenelse{\ifcurrentfield{booktitle}\OR\ifcurrentfield{booksubtitle}%
\OR\ifcurrentfield{maintitle}\OR\ifcurrentfield{mainsubtitle}%
\OR\ifcurrentfield{journaltitle}\OR\ifcurrentfield{journalsubtitle}%
\OR\ifcurrentfield{issuetitle}\OR\ifcurrentfield{issuesubtitle}%
\OR\ifentrytype{book}\OR\ifentrytype{mvbook}\OR\ifentrytype{bookinbook}%
\OR\ifentrytype{booklet}\OR\ifentrytype{suppbook}%
\OR\ifentrytype{collection}\OR\ifentrytype{mvcollection}%
\OR\ifentrytype{suppcollection}\OR\ifentrytype{manual}%
\OR\ifentrytype{periodical}\OR\ifentrytype{suppperiodical}%
\OR\ifentrytype{proceedings}\OR\ifentrytype{mvproceedings}%
\OR\ifentrytype{reference}\OR\ifentrytype{mvreference}%
\OR\ifentrytype{report}\OR\ifentrytype{thesis}}
{#1}
{\MakeSentenceCase{#1}}}