The Planning Portal is the Government’s online planning and building regulations resource for England and Wales. it includes lots of information on the background to Neighbourhood Plans. You can find it here.
As well as the NPPF the Government has also published the National Planning Practice Guidance portal which provides guidance to Planners, Councillors, planning applicants, developers, and the public as to how the National Planning Policy Framework is to be used. The portal can be found here.
The NPPG includes a section specifically about neighbourhood planning. This provides advice on the neighbourhood planning system introduced by the Localism Act including key stages and decisions (e.g. deciding neighbourhood areas, the legal tests for neighbourhood plans, and the process of independent examination and referendum).
The National Planning Policy Framework was revised in July 2018 and February 2019. It sets out the Government’s planning policies for England and how these are expected to be applied. You can browse it or download the full document here: NPPF 2018.