You are very close. Use text()= rather than @text (which indicates an attribute).
e = root.xpath('.//a[text()="TEXT A"]')
Or, if you know only that the text contains “TEXT A”,
e = root.xpath('.//a[contains(text(),"TEXT A")]')
Or, if you know only that text starts with “TEXT A”,
e = root.xpath('.//a[starts-with(text(),"TEXT A")]')
See the docs for more on the available string functions.
For example,
import lxml.html as LH
text=""'\
<html>
<body>
<a href="https://stackoverflow.com/1234.html">TEXT A</a>
<a href="http://stackoverflow.com/3243.html">TEXT B</a>
<a href="http://stackoverflow.com/7445.html">TEXT C</a>
<body>
</html>'''
root = LH.fromstring(text)
e = root.xpath('.//a[text()="TEXT A"]')
print(e)
yields
[<Element a at 0xb746d2cc>]