  In 2003, the magnetar XTE J1810-197 started an outburst that lasted until
early 2007. In the following 11 years, the source stayed in a quiescent/low
activity phase. XTE J1810-197 is one of the closest magnetars, hence its X-ray
properties can be studied in detail even in quiescence and an extended
monitoring has been carried out to study its long term timing and spectral
evolution. Here, we report the results of new X-ray observations, taken between
September 2017 and April 2018, with XMM-Newton, Chandra and Nicer. We derived a
phase-connected timing solution yielding a frequency derivative of
-9.26(6)x10^-14 Hz s-1. This value is consistent with that measured between
2009 and 2011, indicating that the pulsar spin-down rate remained quite stable
during the long quiescent period. A spectral analysis of all the X-ray
observations taken between 2009 and 2018 does not reveal significant spectral
and/or flux variability. The spectrum of XTE J1810-197 can be described by the
sum of two thermal components with temperatures of 0.15 and 0.3 keV, plus a
power law component with photon index 0.6. We also found evidence for an
absorption line at ~1.2 keV and width of 0.1 keV. Thanks to the long exposure
time of the summed XMM-Newton observations, we could also carry out a
phase-resolved spectral analysis for this source in quiescence. This showed
that the flux modulation can be mainly ascribed to the the warmer of the two
thermal components, whose flux varies by ~45 per cent along the pulse phase.
