The book is divided into two parts that reflect one another. The first part is songs of innocence and the second part is songs of experience. The book was originally written with the poems set into individual illustrations also by Blake. My version had those original illustrations a written text of each poem as well and a commentary on each poem.
In many ways the poems and illustrations are very much something that if it was written today, you would expect to be produced by an art student or somewhat talented crafts person. Blake's illustrations are evocative, but he is not a great visual artist. As individual poems, the poetry varies widely in quality. The best poem is the most famous - The Tyger, which truly is one of the great poems. Many of the other poems are basically doggerel (in the non-pejorative sense). Some of the other poems that stand out for me are - The Lamb, The Blossom, and a Dream from Songs of Innocence. The Songs of Experience are generally better being more complex and interesting and often having contrary moods in them. Blake loves short varying rhyme schemes that are like lullabies with frequent discordant notes.
The themes of the work as a whole are primarily innocence and its loss due to experience as one would expect. Blake is a romantic through and through and idealizes the innocence of a child as only someone who has never been a parent can. He rails against modern industrial society and against organized religion. Another key theme is alienation from God. Blake is certainly a Christian, but he sees organized religion and the practicalities of his contemporary society as taking away the innocence necessary for Christian faith. The only poem to question at all the divine nature is The Tyger which is part of its brilliance.
With the exception of The Tyger, which is by any standards a masterpiece, Songs of Innocence and Experience seems to me more important for its innovations rather than the poetry. What we have here is an early incarnation of Romanticism with all its strengths and weaknesses. We also see not a mere collection of poems, but a collection which is organized with underlying themes and movements and which is designed to be read as part of a whole.
I have not read a great deal of lyric poetry and I want to move into this area of literature. I picked Blake as a starting place and it is a good book for that purpose.