[cybmole]

i check for & fix just two things in epubs before sending to kobo
using sigil
1. in stylesheet - remove all line height statements using this regex:
FIND line-height:[^;}\r\n]*;? REPLACE with nothing - check that minimal match is unticked and regex is set to current file
2. again in stylesheet : manually remove any font-family declarations with FIND font-family:

you will them be able to control line spacing and choice of fonts via the device controls

PS assuming you only want to read any given agatha christie story once, you can get some in epub format from your local library ( assuming you have one that lends epubs) - borrow, read, return, no re-coding hassle. if you are working with downloads, sometimes it's better to try a different source than to have to fix lots of conversion errors.... I expect there are many unauthorized poor quality old conversions from scans out there...