Depends on what you call hard. If you really want more or less all - develop craft 5, often what is not available as a book is available as scrolls. And open every damn thing that can be opened during exploration.
If you only want and need some good ones for crowd control especially, none at all. You ight want to do 2 things - visit traders after every level advance and buy off some low level skill books (which are cheap). In my running playthrough the traders did not always restock skillbooks if nothing was bought or they restocked only every x-th level.
Latest at Hunters Edge you will find some of the highest level spells available, notably meteor storm and hail storm guaranteed.
Regards,
Thorsten Haupts
