Arrangment can be learned by studying production and chord theory. For a more partical approch, supplement it with mapping arrangments to songs that you like and writing to those arrangments. Some riffs are catchy and others are mind blowing. Its a matter of fractions. A catchy riff will fit into so many measures, a mind blow riff will fit into a different amount of measures. A mind blowing arrangment will not work very well with a catchy riff and vice versa.

A good book on melodic composition (or theory) will help out alot. But its useless with out technique. Melody without technique sucks and technique without harmony sucks, you have to have both to make a good melody. Solos are just longer and more technical melodies generally.