梅威瑟：
% !tex=pdflatex
\documentclass[oneside,12pt]{book}
\usepackage{lipsum,lettrine}
\begin{document}
% NOTE1: There have text between Chapter and Section, I just want to the first word be dropcaped like below.
\chapter{Week1}
\lettrine{W}{ith} a drop cap, the initial sits
within the margins and runs several lines deep into the paragraph, pushing some
normal-sized text off these lines.
\section{Day1 of Week1}
\lipsum[1]
Chapter one end.
% NOTE2: There have NOT any text between Chapter and Section, but I just want to the first word be drop-caped like below.
\chapter{Week2}
\section{Day1 of Week2}
\lettrine{L}{orem} \lipsum[1]
% NOTE3: How to do this work like above(just let the first word be drop-caped in every chapter)
% through the whole book automatically?
\chapter{Week3}
\lipsum[1]
\end{document}
我能做的如注释1和注释2所示。我只想找到一个最佳解决方案来自动完成这项工作（注释3）。
答案1
下面的解决方案实际上是一个半解决方案，甚至可能是一个蛮力解决方案，它受到这个答案的启发：https://tex.stackexchange.com/a/290567/231952。它需要三个条件。1）后面不能有空行\chapter；2）后面的文本\chapter不能以 开头；3）使用\标准类：book
\documentclass[oneside]{book}
\usepackage{lettrine}
\usepackage{etoolbox}
\def\dolettrine #1#2 { \lettrine{#1}{#2} }
\makeatletter
\apptocmd{\@chapter}{\dolettrine}{}{}
\makeatother
\begin{document}
\chapter{Lorem ipsum}
Lorem ipsum dolor sit amet
\chapter{Dolor sit}
%
Dolor sit amet lorem ipsum
\chapter{Dolor magnum}
% In this case it does not work correctly
\textit{Lorem} ipsum dolor sit amet.
\end{document}
编辑。
第一个限制可以通过以下方式消除：
\apptocmd\@makechapterhead{\endlinechar=32}{}{}
\apptocmd{\@afterheading}{\endlinechar=13}{}{}
该解决方案的灵感来自于https://tex.stackexchange.com/a/16462/231952。第一个命令将 附加\endlinechar=32到\@makechapterhead，以便行尾被空格替换。第二个命令恢复默认值13。
对于带星号的版本\chapter我们也需要这个：
\apptocmd{\@schapter}{\dolettrine}{}{}
\apptocmd\@makeschapterhead{\endlinechar=32}{}{}