Hi Utsav, If you are going for village trekking then booking isn't required as you can stay for the night in the village homestay. For all other trek it is better to go in an organised way. You can find more info in the trips attached below. Keep exploring!
No guide is required, just carry a bsnl sim nd a offlinr map of the area nd study the route well on youtube. thats it. u also do not meed to book any hotels in advance. spiti is never flooded with tourists. u will easily find rooms and homestays.
Hey Utsav, Spiti Valley is beautiful and if you are an avid trekker then guide is not required but if you are doing this for the first time then definitely guide is required since the terrain is harsh and climate changes rapidly there. Be safe, enjoy your trek and share your experiences.. Cheers!!!!