Sacha writes
I most definitely see myself as an amateur too but sort of bridging the
world between the expert - my husband - and the amateur who joined urg
originally - me. I'm absolutely NOT talking down the RHS Encyclopedia
but for the sake of newcomers to gardening, I did think it worth
mentioning that it doesn't have every plant ever found or bred in it.
We get customers here who believe it does and that if a plant isn't in
there, it's incorrectly labelled, either by us or by others. We use it
a LOT, either to show customers photographs of plants they're
interested in but don't know, or to check some detail or other. I
doubt a day goes by without it being used in our busiest months.

You don't have to be a particularly knowledgeable amateur to start
finding gaps in the RHS books - it's not just that they don't have all
the varieties (no book of such wide coverage possibly could) but sitting
down with Chilterns catalogue in one hand and RHS catalogue by your side
reveals a lot of missing genera too. (Mind, not everything in Chiltern
is 'gardenworthy' by conventional standards).

This isn't a criticism, just an echoing of Sacha's point that it won't
be that long into your gardening career before you start wanting
specialist books on the groups of plants that particularly take your
fancy.
--
Kay