I have the following problem: I need to route packets to an IP address (referring to IPv4 only) that is part of a subnet that's directly connected, through another interface, a tunnel to be exact.
For example on interface eth1 setting the ip 192.168.1.1/netmask_255.255.255.0 would create a route to 192.168.1.0/24 with a default metric of 0 (zero). I need to route packets to a specific ip in that subnet through a tunnel interface called tunnel1. Ideally, I would do "ip route add 192.168.1.55/32 dev tunnel1". This would produce another route with a metric of 0 (zero) to that IP.

The question is: how to set up the metrics so that packets destined for .55 would go through the tunnel; and towards the rest of the hosts in that /24 subnet through eth1?

just use the same or lower metric to the one for the local interface. if it's lower it will override straight off, but if it's a more specific subnet, e.g. /32 vs /24 then that will also override it.

route add -host 1.2.3.4 gw a.b.c.d metric 1

Ok, so if I understand correctly when faced with the same metric in two routes that match an IP, the more specific one (that is, the higher number of bits in the mask) has precedence?

Thank you for the info.

Could you guide me to some online documentation resources where the routing decisions are explained in detail?

Correct, routers should always prefer the longest match. Only if the routes are the same length should metrics then be taken into account.