Some books suggest that if two sounds reaches our ear within 0.1 seconds then ear or brain perceive them as one sound and to hear an echo the time difference must be of at least 0.1 seconds. What does the bolded part means? For example if one phones plays violin and another phone plays guitar at the same time and place I will be able to know that two different sounds are being played and not one sound.
It is about the Inter-aural Time Difference (ITD), which is the term used to declare the Time Difference of Arrival (TDoA) of a sound between the two ears. Please note here that this has to do with the same sound and not two distinct sounds. Additionally, the first experiments had been done with simple monochromatic (i.e. sinusoidal) stimuli (I do not know if the phenomenon has been investigated further with complex sounds).
One example which you can easily verify yourself is to use two loudspeakers, or a pair of headphones (the latter is better since you get rid of cross-talk between the two ears), is to duplicate a sound (start easy with simple sinusoidal sounds with somewhat sharp attack and release times, or use impulsive sounds) and gradually introduce delay to one of the them until you hear them as two distinct repetitions of the same audio event.
It is important to state that the transition from the perception of a single event to two distinct events is not instantaneous after a specific delay value is exceeded. After a delay value, the perceptually single event will acquire a harsh sonic character and after the delay is increased more by an adequate value two events will be perceived. After the delay is increased more, the harshness will start to go away
Note that the delay value that will lead to the perception of two distinct sound events is not a constant value and strongly depends on the spectrum of the sound, its amplitude envelope (which is related to its spectral content), other sounds that may happen to be audible at the same time and a lot more. The phenomenon is highly non-linear and it cannot be easily described with a single number like the $0.1 s$ that you provide (which is already a quite long delay and will most probably result in the perception of two distinct audio events).